You might ask on this forum for Lexingtons, Here's a switch someone wondered about, turned out to be mirror heater switch. It's a 12 v switch.
What does this switch do? - Forest River Forums
I forgot to mention that if the switch in your case is for an electric element for the water heater, it should only be turned on when the water heater is filled with water. The element will burn out if tank is empty.
To use a 120 v light switch to power a 12 v function (ceiling light, porch light) would be a modification by a previous owner, I've not seen such installations by a manufacturer. A battery isolation switch would be a momentary switch to trigger a solenoid.
Here's a link to the 2015 owner's manual, perhaps it contains information you can use:
Forest River Manuals