Personally, I would not want to go down the road with the camper supported by any less than ALL of the points where it would contact the truck bed.
I think if you go pounding down the road with the camper supported by a rail across the front, a rail across the back, and wherever it happens to make contact with the truck frame, you will be in for a rude surprise at your destination.
If cross members are added every couple of feet, how would it differ from a TT, 5er, Class A or C? Are they not basically a TC, sitting on a frame with crossmembers?
A TT, 5er, or Class A/C coach is designed and built to sit in a ladder frame.
A TC is designed and built to sit on a 4'x8' platform.
I suppose if you put these cross members in the EXACT RIGHT locations, and don't allow the TC to shift AT ALL, it might work. Your truck cross members would need to align perfectly with the TC cross members, or else they will punch up through the floor of the TC in the "soft spots" between cross members.
Remember you're talking about 1000's of pounds hitting HARD here, truck going over a pothole, KA-BANG, not a 200lb person walking across the floor.
I don't agree that you need to sit on a deck. My TC sits on 4x4's (sometimes 2x4s), which are side to side in the bed of my truck. There is one in the front, one in the rear, and one on each side of the wheel well. I have well over 125,000 miles and seven year with it this way, on many roads in several states. Absolutely no problem at all.
I had my previous TC setup the same way, with no problem.