f350 electrical issue

Hi.  Have an '06 F350 diesel.  Not driven too much.  Battery was dead.  Charged 4 hours.  Took to the auto parts store and checked the batteries.  They said they were ok, just needed a good overnite charge.  Put it on the charger overnight, cleaned the terminals, etc.  Next day, Started fine.  Ran 2 hrs, was ok.  Nxt day, started fine (temp below 32 degrees here), checked all lites on the trailer, everything fine.  Towing 3 horse slant 5th wheel trailer--EMPTY.  Driven about 30 miles, stopped at a stop sigh, went to back up to let another vehicle thru a tight intersection and had no reverse.  I had lost all dash gauges, interior/ext lites, radio, pwr windows AND reverse.  Engine still ran fine.  Engaged in drive, ok.  Neutral, could rev engine.  In reverse, NOTHING.  Couldn't rev engine, nothing.  Drove about 5 miles, gauges came back on, lites, windows and reverse was back.  Checked all terminals on the battery for a loose connection, checked all interior fuses (under the steering wheel) associated with gauges, all were good.  Drove another 30 miles with no repeat...  Any suggestions?  I don't want to have to try to figure out how to turn this rig around again with a 34 foot trailer on it and no reverse....Thanks.