If you posted some photos of the inside, we might be able to give you a few pointers.
My 1988 Bigfoot had no provisions for batteries. It did not have a battery charger, converter only. You connect a battery to the converter, it would fry the battery. I added a 55 amp Iota charger/converter and a single AGM battery inside the TC. I tried the batteries in the wheel wells, and did not like it for several reasons.
This is where I installed my AGM battery. It has been there since about 2006. There is room for two AGMs, stacked. I have needed but one.


I mounted the Iota here.

I also added a TriMetric and solar, and ran the wires here.



Wayne