OP, Ok, I get your no sag with foam concept.
another idea instead of unistrut which is heat, corrosion, conductive we used pvc 'lumber' on a tt we modified for wintering over in Canada. Solid PVC , such as 'brick mold' fastened to the frame with sdst screws, 2" foam then the coroplast with wood screws to the pvc.
Pvc can be bought anywhere as dimensional lumber, shaped like wood has insulation value, without rot.
For heat tracing we used cut to length ss 5w' cable, old waterbed heaters under the tanks , all routed up to where we could plug them into the inverter when traveling or into a shore power plug .
