What all does the Accessory Battery power?

I have a 1998 Ford E-150 Explorer Limited Special Edition conversion van…My 12 V vacuum and the power couch work at all times.
My dome lights, inverter plugged into my 12v power port, original TV (I think) and other accessories only power on when my ignition switch is in accessory mode… Is this normal?
I’m wondering, because when the accessory switch is on, is the accessory battery, mounted underneath on the frame, being used or the starter battery? And how can one tell?

I’m asking because I would like to add a new TV, DVD player and a few other items, and I would like to make sure these items all run on the auxiliary battery, and not drain my starter battery…

The van appears to be all original, and I’d like to keep it that way, so I’m not interested in adding other batteries, solenoid‘s, relays, and so on…

I want to run it the way it was designed, and will only have limited accessories, such as dome lights TV and DVD..