Different trailers seem to have different setups! On ours, the inside water heater switch (blue light) controls the propane water heating only. To use electric heat a small rocker switch in the water heater compartment needs to be turned on. For really fast heatup, both can be used together. Just have to make sure the electric heater is off before draining the tank!
Tina & Ted & pooches
2012 Ford F-150 XLT 5.0L V8 4x4 with tow package
Andersen WD hitch with sway prevention
2012 Rockwood UltraLite 2304s trailer