I bought my truck and camper mated together. It had previously been mounted together by an RV service place when the owner replaced his pickup.
There was one thin layer of rubber mat, the two pieces of plywood, then the fiberglass camper base sat on all of that to shim up the camper for cab clearance.
The camper had slid on the plywood, and the sheets of plywood had slid in relation to each other, assuming at mounting the plywood was near square. The camper had slid back approximately 3 inches somewhere during all the sliding. The previous owner had hauled horses to horse shows with the combination, so no off roading that I am aware of. However, in the PNW we have a lot of hills our highways climb and descend. Some are steep, and some of these older freeways are now rough and "Bucky".
Tie downs are Torklift frame mounts front and rear, which means there is at least a couple of feet for the turnbuckles to bridge. In this situation non sliding (grippy) shims would be best.
I remounted with a thick stall mat, then a sheet of that plywood, then the same thin rubber mat. Seems to be staying in place for me, but I don't have 27 thousand miles on the combination yet, either.
My experience. Might be completely different for everyone else.