I use a combination of wood and 2 10-packs of plastic blocks. I use 3 2x8 scraps + a brick under the tongue jack. First time out, I used the plastic blocks to level side x side, but found it took so many that I didn't have enough for the stab-jacks. When I cleaned out my garage to sell the S&B when I went "full time", I found a 12' piece of 5/4 x 7 PT deck board. Cut it into two pieces w/45-degree ends for under the wheels, cut 2 grooves in the bottom piece that mate with 2 1x2 "tongues" that I screwed to the under-side of the top piece. That way they "nest" together, so driving onto them doesn't kick one out from under the other. Then I put a coat of deck stain on them (as much for looks as for preservation). Sometimes, I only need the bottom one; sometimes, I use both - depending on how out of level the campsite is. That leaves all 20 of the plastic blocks for under the stab-jacks. I'll use 2-6 under each jack (depending on the front-to-back slope of the pad) to keep the stab-jack from sinking into the soft sand, dirt, or asphalt pad. That enables me to keep my stab-jacks extended less
than 1/2 way out for improved stability.
That what works for me . . .