I had a similar problem, one of the rubber ends on the rod of a height valve had broken and the rod was dangling. Replacing that bit of rubber put my RV back level. On a 3 valve system, typically rear left and right valves control side to side and rear height, front valve controls front height. The side that is not inflating is the side the valve isn't working. Make sure RV tires are on blocks so it doesn't drop and crush you while underneath. If air pressure is up in tanks, then go underneath, disconnect the rod (8mm wrench) on the valve not working and move lever down. If air doesn't inflate bag, that valve is bad. Remove, take to truck parts place and buy a new one. Make sure you have correct measurements between frame and axle to set the new one correctly. I cut a dowel to proper length to make measurement easier while underneath.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI