If the switch has a red light to the left of a rocker switch and the light is marked reset. It is likely the water heater gas control switch. If it is like mine, the down position should turn on the light and the water heater should come on.
However, the water heater takes 12 volts to work so the light nor the heater will work if there is a problem with the 12 volt circuit.
You note that neither your pump nor control panels work unless you are on shore power. That is an indication you have a problem with your batteries. Either the batteries are dead or they are not connected. These items works when you have shore power because the inverter/charger is supplying power and the batteries are not needed.
If you have a volt meter check the voltage across the battery terminals. Less than 11 volts and the batteries are discharged, check the charging circuit. You can also use a 12 volt test light. They are inexpensive, but less accurate. You can get one at any auto parts store. Just touch the light across the battery. Bright light battery ok, dim light battery discharged.
That said, I would bet the battery terminals are corroded or there is a lose connection in the wires. I would disconnect all battery cables one at a time and clean both ends with a wire brush till they are shiny. Reconnect and your problem may just go away.
Hope this helps.
P.S Do not overlook the battery ground connection. The battery need both a positive connection and a ground connection to every appliance or the appliance will not function.
My wife and I would travel full time, but we have to sleep occasionally.
2001 Tiffin Alegro Bus 40 ft DP named the Wild Eagle
Followed by the Eaglet - A 2005 Jeep Grand Cherokee.