you should have a switch on the outside. Just turn it off. Make sure you do, because if the element melted and bent, it could touch the sides of the tank and cause a hot skin condition.
Once you engage the switch on the inside for the gas, it will work on gas only. at least that's how mine works.
EDIT: and read up on RV hot skin condition. test it before you accidentally harm yourself.
EDIT #2 for safety: flip off the circuit for the HW heater to be safe.
2019 Heritage Glen LTZ 272RL
2018 Ram 2500 CTD