OK, BOTH inside switches are 12 volt. NONE is 120 volt. Behind that rear black box is the 12 volt activated relay that turns the Black wire of the 120 wires ON to the 120 element. The YELLOW wire on the outside at the Ignition control module is what is HOT (12 volt) from the 120 red lighted switch in the RV. Inside that black cover are 3 parts.
1. 120 element
2. 120 12 volt activated relay
3. 120 volt water tstat.
The element comes out from behind and is about 12 inches long. Doug
PS. That yellow wire is butt connected to a red going into the black box
