i have the same issue. well, what'ser'name and i have it.
we also had that same issue with a memory foam pad on the bed in the house.
she bought a roll of that tacky stuff that keeps rugs from sliding around on the floor.
put a piece of it under the memory foam and it hasn't moved an inch since.
we'll be taking some remnants of that stuff with us when the weather warms up. i'll report back eventually. i HATE cold weather camping.