We have the same problem. With the spring snow melting, our gravel parking spot is turning to mud. So that's exactly what we did - backed up onto pieces of 3/4" plywood to keep the wheels from sinking into the mud.
Make sure you have a spotter and go slow, we had one of the plywood pieces flip up a bit as the tire went on, but it was fine once the wheel was in the center.
2012 Fleetwood Tioga Montara 23B
No Slides...No Toad...Just 24' on the Road