Single side adhesive acrylic foam, like the kind that you stick on a pickup truck bed and then set the canopy on, is a pretty good insulator.
I used a combination of wood strapping going the length of the bus, and rigid insulation board tucked behind and flush with the boards (2" + 1", total 3") I just used hex head self drilling screws to attach the strapping to the steel frame.
The boards held back the 2 inch foam, and the 1 inch was taped with double side carpet tape to the other foam, and then a mylar hvac tape was applied to seal the seams between foam panels and to the wood. Over each screw that holds the wood strapping to the steel frame is a square of that acrylic foam tape, then applied a small strip of the mylar tape over that (without compressing it too much)
As for running t&g boards the same direction, I'd suggest buying some metal strips, attaching those to the wood straps, so you have a criscross with the wood acting as the thermal break.
Perhaps you could use those long aluminum strips for transitioning a carpet run? They are flexibile and have some rigidity. Install one every 16 inches, they will be the strapping substrate for the t&g. Wherever the t&g lays on top of a wood strip, make sure you add material to keep from pulling the two layers together (the same thickness as the aluminum strips) and it should be pretty strong.