[JRTJH]

In almost all models of RV air conditioners, there is a "freeze plug" which is actually a thermistor that monitors the "fin temperature" and automatically turns the compressor off/on to prevent the ice buildup in the evaporator coils. You can usually see the "freeze plug" by removing the filter and using a flash light to shine up into the surface of the evaporator fins. It is a white wire that connects to the control box and the distant end is "stuck into the fins" about 1" below the top of the evaporator. It's the only thing that's "stuck into the fins" so no confusing it with something else. If it has vibrated out or is loose in the fins, it will cause the ice buildup you're experiencing. If it is loose or unplugged, reinstall it in the evaporator fins. If you want, pull it out, wipe it off and then reinsert it about 1/2" to either side of the original location.

All of the previous reasons for icing are valid and common, but nobody mentioned the "freeze plug"....