Helen, when you say ignition on, don't you mean accessory (left) position? Or only with ignition ON?
Try placing the ignition in ACC and see if the sunroof will stay open. In this position the power windows should operate normally, and the radio will work. Maybe the TV will also work.
But given the constraints and situations you describe here is what I would do, and none of this will jeopardize the vehicle warranty or require much work:
I assume the van has a receiver hitch...if not, I would install one. Be sure to buy a locking pin, and a 'hitch tightener'.
Buy a good-quality hitch rack (with curved corners) and set it up to carry an additional deep cycle battery and that battery placed in a nice marine type battery box, secured to the rack. A battery box will be low enough to not block the rear doors. A low profile plastic tote could be added for extra storage if desired. Or a small pile of firewood!
Within the battery box, mount a small, sealed, on-board battery maintainer with an AC plug. This is for when you have an outlet nearby.
Route small cables and sockets into the cabin for powering the TV, laptop, whatever.
Also I would buy a small 50 watt panel for the roof of the van, along with a small PWM controller, to balance the parasitic loads of the van systems (ECMs, interior lights, power windows etc). This will be wired to the starter battery.
Optional, and I would recommend, a portable 100 watt folding 'briefcase' style solar panel and controller, with battery clips or connector, to hook up to the auxiliary battery when boondocking, if sunlight is available.
If the starter battery ever failed to start the van due to overuse of the van's systems OR no usable sunlight for a few days, the secondary battery could be used to jumpstart the van.
This is what I have done, (with minor changes) and it works.