UPDATE: Temps were back into the 20's last night and the mattress was significantly warmer than it has ever been!! Installing the Reflectix on the bottom of those 2x2s created just the right "dead air" space under the plywood platform to provide excellent insulation. I don't think adding foam under the Reflectix will do much, it's the air space that is creating the insulation.
The sides of the bed platform are also no longer cold to the touch. What I need to do now is get some foam up under the counter tops of the nightstands! I'll have to remove the drawers and see how much space I have, likely won't get to that until we get back home...
Frankly I was surprised what a difference this made, and I would recommend that anybody that camps in colder temps consider this mod...it was pretty quick and easy. The foam panels actually came the exact width for the bed sides, all I did was cut them to length.
Unfortunately I have a bunch of stuff in the underbed storage, so I can't cut off access altogether. I did replace the door seals over the summer so I know the weatherstripping is tight. Adding some foam to better insulate the back side of those doors is an excellent idea though!