According to the Atwood owners manual found here
You have an electronic ignition gas fired water heater with electric heat option.
There apparently is no "hidden" switch for the electric heat on your Atwood.
(Suburban water heaters have this hidden switch found behind the outside access
You should have a dual switch panel somewhere in your trailer.
It will probably look something like the picture of a "dual switch" found on page 7
of the manual noted above.
The owners manual also shows a single switch with another light beside it.
You could have either configuration.
If you have dual switches one is for gas and the other is for electric.
If you have a single switch, apparently it turns on both types of heat at once??
That doesn't sound right.
Read the manual and maybe someone will answer who has your exact setup.
[I hate it when they print a manual that covers all the different options. It's up to
you to try and figure out which part of the manual applies to your unit!!]
If it is functioning correctly, you can have gas or electric or both at once to heat
Most of us use the gas only if we are in a hurry and use the electric heat to save
the LP gas other times.
Also the electric heat is nearly silent while the gas can be a bit noisy in the middle
of the night.
Hope this help!