I have not had to get into mine. But pictures in the link below show the J box to be at the top left behind the front panel. Depending on your location you may or may not have access to the back without removal.
https://www.pplmotorhomes.com/parts/...heater_09.0073
Also, mine is older and doesn’t have an inside switch. If you have an inside switch for electric then your WH will have a 12 v relay to switch on/off the 115 vac. The pictures in the next link appear to show additional wiring for that.
https://www.amazon.com/Suburban-SW10.../dp/B00FA4J8A4