On both of the RV's I have owned, there were two rocker switches for the water heater. One switch said electric, the other said propane. As John mentioned above, if you haven't used that feature for a while (or ever), it may take a few cycles to get the propane flowing through the line properly. If you turn the propane rocker on, you should be able to hear the water heater clicking as it tries to light the flame.