Here is what I did to insulate my camper.

5/4x6 treated decking with 1" foam board "chewy center". Much lighter than plywood and insulates. The wood perimeter solidly supports the camper edges.

Insulated compartment doors.

Insulation attached to the tub sides and up under the wings.
I have also installed insulation inside the TC on the walls inside the cabinets and appliances where I was able to install it.
The insulation made a great improvement when winter camping.
Also get a couple cheap rugs for the floor. It's warmer than vinyl.