I have a set, they're great on certain surfaces. They will settle down in wet grass and soft ground depending on the weight of your RV.
I had one crack in half but it got replaced for free under warranty. I used them in a site with crushed rock. The rocks did not allow the RV load to concentrate across the face evenly. Therefore created high load points and caused them to crack. I now use them only on grass, sand, and concrete pads. I avoid use on rock pads. I think if one puts down a wood pad first then the leveler on it that would prevent the leveler from cracking. I haven't had the opportunity to try this yet, so it's only theory at this point.