Lynx Levelers seems to work pretty well, although they are only about 8-10" square. We have two bags of 10 in our FW and use them for the front and rear jacks and to level the FW as well. If you can find them on Facebook, sometimes they have a giveaway of various things.
2012 GMC 2500HD D/A Crewcab
2012 Jayco Eagle SuperLite HT 26.5 RLS
2009 Crestliner Superhawk 1900
2013 Polaris Sportsman 550 EXP
2008 Polaris Sportsman 700 X2