Sounds like you have an additional ON/OFF switch for 120V AC electric element has been added into the circuit probably by previous owner ("(looks like a white 120v light switch"----mfg. would have just used a rocker switch)
Suburban's come with an ON/OFF switch in outside compartment lower left corner.
Up to RV MFG. to install another one OR not.....lots don't
BUT same holds true for Atwoods prior to 2004..they had an ON/OFF switch on cover over electric element on backside of water heater.
Depending on access to backside an additional ON/Off switch could also be installed.