Some folks have knee jerk answers to everything....
Now to the topic.. If I am filling in the left out parts.. Your water is no longer working on electric.. But still works on Gas.
Suspect list:
Electrical supply to water heater (By the way folks Mine is NOT "Just plugged in" it is hard wired) Starting with the park pedestal.
Then we have the breakers in the RV. IF you know how measure voltage
Now we have the relay and some side stuff I'll get to in a paragraph or 3
And the heat element
And all the wires that connect..
My Favorite suspect here is the heat element.
The side stuff includes the thermostats and control board. Those are #2 and 3 on the list.
IF. and I stress IF it's the control board.. Go with Dinosaur boards.
The relay, though suspect... Is kind of down the list.