I use 24" pieces of 4x4 laid side-by-side under each of the three jacks. A couple of 2x8x16" are for when one side of the coach is lower than the other side.
Our coach tends to move forward a bit when jacks are deployed because the rears always touch the ground before the front one does (even with tire chocks), so I need more than 1 foot of length for the jack pads.
4x4s are pretty cheap, too, and easy to replace/clean.
Rick and Sandy
2003 American Eagle, 52K miles