The switch outside is to turn on the electric hot water element, the switch on the inside is to use the gas part of the hot water heater. If you are camping where you have electricity I would use the electric element and save your propane, if you have no electricity use the propane. Make sure the tank is full of water before turning either on. Run water through the hot water valve furthest from the water pump until you get a steady stream the air should be all out of the tank then. If you don't get the air out the electric element will burn out and you will have to replace it. Once its full of water turn outside switch on and you will get hot water in about 15 min. If you have no electricity open the propane tank valve wait a few min. then hit that lighted switch inside and when the light goes out the heater should be lit. Make sure when you pack to go home you turn both switches off. I think I got it all the right way regarding the propane switch itself the rest I'm sure of.
Think about things before you do them make life easier not harder.