Easier method. If you have a onboard Amp meter and use electric to heat the water when you first turn on the electric the Amp meter will show about a 10 amp increase.
When it drops back down the water is hot. Also works if gas and electric are both used.
For my Minnie that has only gas heating I installed a mechanical temp gauge on the outside of the tank so I can see the water temp. And can no longer hear the gas on.
2015 Fleetwood Discovery 40E on a Freightliner XCS chassis with a Cummins ISL9 pulling 1 and/or 2 motorcycles, '07 Honda Accord OR a 17' Runabout Boat.