No reason that wouldn’t work. It’s cheap and easy, slap a rubber mat on top of them and if the camper moves around more than you’d like, revise the plan. Although if they are wedged tight to the wheel wells they shouldn’t move.
Maybe consider a thin sheet of plywood to tie the pallets together. Few screws that you could remove to take the pallets out individually.
2” thick 40psi foam board is about $50 a sheet so $100 saved if you can use the pallets.
Foam is readily available though at building supply places
