Personally I won't level with any tire off the ground, structurally I do not feel it is safe.
The owners manual for our 1990 Bounder had instructions for building the three level ramps discussed earlier in this post. They worked well for me. There was a little problem, however, they were darn heavy. That is why we have a RV with hydraulic levelers.
That said, all the examples here deal with FRONT of the RV being low. The manuals for the Bounder and the Winnie note that when leveling when the rear is low, that blocks must go under all the rear tires not just one on each side.