[Jeff n Susan]

I use 2x12's also, but I have also learn how to build cribbage from firewood! A few times that was necessary when I didn't have enough blocking!
We carry a plastic tote for the blocks and most of the time I have a few left over, so I just put them under the jacks to shorten the stroke and make the camper more stable. It's all in what works for everyone in their own situation.
Good luck!