As part of an upgrade, the previous owner paid some serious dollars to have installed the HWH #725 hydraulic leveling system. (This was just before they decided that what they really needed was a larger RV.) The HWH system was repaired under warranty after we acquired the RV. Aside from that, along with consulting every HWH manual I have been able to locate, the system has never been reliable. Concerned that the levelers would become stuck in the extended position(*), we now resort to the orange lego blocks. Low-tech but also foolproof. As suggested by others. wheel chocks are an excellent investment.
(*) Even though the first page in the HWH binder of manuals are the instructions for manually retracting the levelers.