The outside switch is meant to be a "safety" switch to prevent you from inadvertently turning on the inside switch while the tank is empty (which will burn out the heater element)
Whenever you drain the WH, turn the outside switch off.
When the tank has been filled, you can turn the outside switch on and then control the WH with the inside switches.
BTW, look closely at the outside switch, it is usually mounted upside down (up is off - down is on). There may also be a small spring clip through a hole in the switch that is meant to prevent accidentally turning the switch on.
To add to the confusion, some trailers only have one switch inside (for gas) and the electric portion is controlled by the outside switch only. If you have two switches inside, you will still have the outside switch.
