Yes, the Compacts were originally constructed with two interior support walls, which in the Juniors fell right at the rearward edge of the wheel wells. They do need something to support them there; it firms up the body and keeps the roof from sagging.
When I stripped down the interior of mine in order to remove it from the frame, the support walls were 1/2" plywood, affixed through the roof of the body by a couple of screws, and screwed to the surrounding cabinets and floor. Personally, I will be reinstalling mine when I get around to the interior work. However, I will not be screwing them into place through the ceiling, because I like eliminating body holes wherever possible. I am going to take a page from what the original manufacturer did with the bed supports, and simply fiberglass my cabinet framework to the interior of the body, thus eliminating several holes where leaks
As far as no ensolite, that started in (I believe) 73, so your 72 should not have ever had it. Personally, I gave mine a good sanding, a coat of high quality bonding primer, and then painted it with a good oil-based paint
. If you want to cover the walls with something, I would suggest a marine wall liner. It's easy-ish to install, and it will give you some insulation and condensation resistance. Plus it comes in many nice colors.