2x4 cribbing, something like this:
I made mine 18" x 18" with three 2x4s per layer. Little more flexibility with height than cinder blocks and they'll handle mobile homes, park model trailers, modular houses, etc so plenty strong enough for an RV.
Tried using tripod stabilizers/jack stands and was never able to get rid of the sway, set the trailer on 6 of these and it's like being on a foundation.