I get 12.0 on the starter relay trigger wire with key in crank position
I get 4.0 +/- with key in on position
The fuse #5 is good

I see there is no starter relay in the fuse box(power distribution box) like some models have. so that can't be it

also the fuse diagram is confusing as it says fuse 5 is just some lights, but fuse 10 says see 15 and 18 for starter relay ???

 

Here's the starting circuit flow:
Battery -> Junction Box -> Fuse Box (fuse for ignition) -> Ignition Switch -> Fuse Box (fuse for range sensor) -> Range Sensor -> Starter Solenoid -> Starter Relay -> Starter Motor

Ford uses "starter relay" when it should be called Starter Solenoid, it's fender mount.
The "Starter Relay" shown above on the flow, that's attached physically to your starter motor.

Some Fords use fender mount, some other Fords use fuse box type, that's why you can't see yours in the fuse box.
The Starter Solenoid that you tested the R/LB wire at the S Terminal shown on the pic below it should look like this:


So the trigger wire circuit is GOOD up to the Starter Solenoid from the battery (look at the flow above) at the very least.
Fuse 5 description says Backup Lamps, DRL, Trailer Battery Charge Relay, in reality it's missing the "Range Sensor" on the description. The range sensor hardly goes bad, if your starter solenoid goes bad, it would blow the Fuse F5's 15A fuse. The starter solenoid gets its ground from the metal base where it attaches to the fender, when at rest (not cranking, solenoid not energized), your R/LB wire is grounded to the same spot at the fender, when energized (cranking), you get 12V (look at the flow above). When the starter solenoid is bad, it sends ground to your R/LB wire when cranking that's why fuse 5 gets blown. You said fuse 5 is good, you save yourself a lot of time. Also don't worry about F10 or other fuses they are meaningful in your case when F5 is good.

Go back to the starter solenoid, there are 2 thick red wires, one goes to the starer relay (attached to the starter motor), the other is feeded from your batteries. Identify which is which. One should be always hot, the other is hot only when cranking.

1) The one that's always hot, get the voltage there and compare with the voltage on your battery, are they exactly the same or very close, what you get on both?

2) The one that goes to the starter motor, you have to crank to get the voltage, and what voltage do you get?

While doing the 2 tests above, do the terminals or cables look really bad visually?

 

interesting
12.6 batt
12.6 hot side
11.58 - 11.68 with key in crank position on the downstream side

so internal resistance in relay but if that's all it was I'd think directly shorting it should have cranked it.
terminals and wires didn't look bad but I'll clean em anyway.

 

Above pic is the starter relay (attached to your starter motor).
The red arrow, the 2nd trigger wire, this time on the starter relay, not the solenoid you tested previously, you said you get 11.58 to 11.68 on the fender starter solenoid, so you should get exact or very little drop of voltage on this.
The blue arrow, the direct feed from your (R) battery with 12.6V. So you should get exact voltage (no cranking needed). If not, the connection is bad.
Yellow arrow gets 12V when energized (blue connects to yellow), check what voltage you get at the terminal (yellow arrow) when cranking.
Check pic below:



While you are at it, check for the ground terminals for corrosion and do some wire brush cleaning if needed. The starter housing is the ground and wherever it is bolted onto.

 

trying to clip a lead onto the red arrow wire but it's hard to see, doing by feel cuz it's on top and inaccessible but I think I'm on it and I get - with key in crank position - 10.58 - 10.78 (starts low slowly climbs up while holding key)
it even looked like the hot wire (blue arrow) was shorting against the "yellow arrow" conx so i tried to bend it away
might have to take starter out to get a definite lead on the red arrow but I assume i'm on it since I do get a signal with key to start pos

checking blue arrow w key off 12.51v

yellow arrow w key in start pos 11.54 - 11.60

it got dark, have to get back on this tomorrow am

 

No need to test further as long as the terminal at yellow arrow is non-zero, that means the starter relay at the starter motor is functioning. That also means the red arrow of the starter relay at your starter motor is receiving 12v from your fender mount solenoid (when energized) thru the thick red cable.
The terminal at blue arrow is a big problem when you get only 12.51V and the battery with 12.6V. Please check one more time to confirm.
1) get a reading from battery (right), get the reading too but battery (left), let say they are 12.6V still
2) get the reading of the terminal at the blue arrow again if it's less than 12.58V you just have to clean all the terminals at both batteries as shown in below diagram:

And the 2 arrows above in magenta, deep clean them. Clean all 4 grounds: G101, G106, G107 and G108 too.
G101 Below:


G107 Below:


Then get the reading of the blue arrow at the starter relay, make sure the voltage is exactly the same or 0.1v or 0.2v drop (12.51v vs 12.60v is not acceptable), if not, then you have to replace that entire cable shown above in magenta color.
At least your ignition switch, your range sensor, your starter solenoid and your starter relay are good.
You said you are trying to sell this truck, so don't need to be a perfectionist and beautify all the wires, just replace what's necessary and don't throw any parts at it. Once it starts good, do a couple I/M readiness drive cycles and get all the monitors ready then just slow charge both batteries one at a time, then put them on trickle charger and that's it.

 

I believe it is time to clarify some info.

A relay just relays power from one place to another.

A solenoid not only relays power, it also does something. That something is it MOVES the starter drive gear into the flywheel to crank over the engine.
Some starters rely on the spinning effect of the starter to send the drive into the flywheel. Those starters ,older type, don't use a solenoid on the starter.

Now that we know that,the ,trigger wire , if you want to call it that, activates the solenoid which activates the starter motor.

Full battery power must be available at the starter top post of the solenoid. When the "trigger" wire gets power from the ign sw path it not only activates the solenoid, but also physically moves the drive gear into the flywheel.
Poor connections, corroded cables and defective starter can all prevent the starter motor from operating and doing it's job.

The better thing to do to troubleshoot the system is do voltage drop testing. That will check connections , cables and components
There are several videos available if you want to perform the checks.

 

Thanks again for all your very helpful diagrams and explanations!

when you say " make sure the voltage is exactly the same or 0.1v or 0.2v drop (12.51v vs 12.60v is not acceptable)," ,
do you mean 0.01 or 0.02v drop? 12.51 to 12.6 is 0.09

 

If you still get 12.60V at the battery, but the terminal at the blue arrow you get 12.58V or 12.59V that's barely acceptable, you get 12.57V or below after deep cleaning you just have to replace that bad cable. This time you know no cranking no key needed. Please note you are not even testing the resistance with ohmmeter. If the terminals at the battery are really bad and can't be cleaned, try to cut the terminal and replace just the terminal if the cable length allows you to do that. Personally I use hydraulic crimper with bunch of different terminal sizes because I make my own cables all the time. Your red cables should be either 2awg or 4awg. No car makers use thicker than 2awg unless it runs a long distance.

 

You just did part of a voltage drop test.
The missing part is you need to measure that voltage with the circuit under load, that is attempting to start the engine.

The cable could be corroded inside where you cannot see it.

Once you understand what you are doing when doing voltage drop testing , you will find it very useful.

Check the videos doing, "Voltage Drop Testing". Too much to write here.