I have used the foam before with my previous Lance camper to clear the fifth wheel brackets in my truck. I was initially using plywood with a thin rubber mat but decided to give the foam a try for weight savings and more insulation under the floor. I noticed a little bit of movement fore and aft when driving down the road with the foam. It wasn't much but I could see the nose of the camper moving(bouncing) more than with the plywood. This was with 3/4" foam.
With my current Northern Lite I have built a 2x2 framed plywood floor to space it up. I won't do the foam again. I did like how it weighed next to nothing though.
This is my experience. I believe others have had good success with the foam though.