If you want to use 110v only, don't activate the switch on the wall but make sure that the small, black rocker switch located at the bottom corner of the outside access compartment is in the ON position. If you want to use propane, press the inside wall switch to ON. A red light will come on momentarily and then go out when ignition has taken place at the tank. You should be able to hear it when the propane "fires up"
If you want to use both for faster heating, follow both of the above steps.
This is how our HW tank is set up - yours may be different.
2008 Cougar 5th Wheel 27RKS
2005 2500 GMC Duramax