Some of your questions are now year/model specific - did you buy a 95-97 RT 190 Versatile?
Just wondering. The OEM aux battery is located in the dinette seat box on the driver's side in the '95-96.
On the '97 it's nearer the rear of the vans, Dodge access is from inside the rear doors, under the carpet,
on the Chev from the outside and underneath the van. I'd guess that if 2 6V batteries fit in the same
physical footprint as a single 12V deep cycle battery, you're in business.
You could try asking Roadtrek Service about how many might fit. See more at my p.s. below.
No idea where you'd put an inverter but I'd put it somewhere near where you'd be most likely to use
it. In my case I removed the OEM TV from the entertainment cabinet, and use it now as extra storage
and as "charge central" for my electrical gadgets, like the laptop, camera batteries, cell phones, etc.
can be connected to a 150W inverter and I run my TV off of a small separate dedicated 100W inverter.
p.s. If you did buy a RT, I'd suggest you try sending your questions regarding adding/replacing
electrical stuff to firstname.lastname@example.org
- there's a gentleman there named Peter who is probably
the most knowledgable person regarding replacing or modifying OEM stuff. He might be able to
answer your questions regarding batteries and inverter placement as he seems to have blue
print level familiarity with the size of the areas you mentioned, and what is safe and not so
safe to do.
You'll need to include the VIN of your van so he can give you a model specific answer.
He's quite good, and many of us have asked him our techy questions.
Or, many answers can be found in the various owner's manuals from Roadtrek. They're free,
in PDF format, and viewable/downloadable - here's a link to the RT manuals page....