That switch in outside compartment is for electric element......only.
Now you could have another switch inside that is for electric also.
If you do then BOTH have to be on to heat with electric.
The outside switch is OEM (by Suburban).......the RV mfg. sometimes adds another switch inside for convenience so you can leave the outside one on and then turn inside one on/off for electric heating.
The electric uses 110V AC power from the AC Panel (circuit breaker)
To heat with propane you will have another switch that is for the propane only.
The propane uses 12V DC power from DC Dist. Panel (fuse)