There should be a 4 wire low voltage cable going to the hot water heater.
3 Yellow , 1 white Sorry can't say which yellow is which but you can look at wiring diagrams on WinnebagoInd.com if you don't want to do trial and error
White = Ground
Yellow #1 = 12 volts towards water heater when calling for 120 volt water heater element on
Yellow #2 = 12 volts towards water heater when calling for Propane water heating
Yellow #3 = output from hot water heater, 12 volts out only when there is a problem with propane water heating, where the flame did not light after 3 tries to spark ignite it. Goes to a fault indicator in or near the propane water heater switch.
If you have a pilot light type water heater you may not have #3.
Randy - Manhattan, Kansas
2015 Vista 27N
2016 Honda HR-V