Switch left on now Atwood RV electric water heater will not heat. What needs replaced?

Sounds like selector switch is bad. I would start there first. If not the switch, heating element probably burned out.

If they are on outside of tank , I'm thinking your new tank has motor-aid feature as well. That is used to circulate coolant through from engine of motorhome while driving and it keeps water heated. That feature usually adds to price of tank. Hope you didn't pay for some feature you"ll never use.

http://waterheatertimer.org/Atwood-water-heater-resources.html

Gene
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Copy following link for Atwood water heater resources:
http://waterheatertimer.org/Atwood-water-heater-resources.html
http://waterheatertimer.org/How-to-test-water-heater-element.html
Add a comment and say what you discover

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

I do not have specific answer for this problem
Open following link to identify product, download service guide, parts list and other resources:
http://waterheatertimer.org/pdf/Atwood-Water-Heater-Service-manual.pdf
http://waterheatertimer.org/Atwood-water-heater-resources.html

Directly across from yellow wire.

The two switches control the water heater either in the gas or electric mode. The switch with the lights would be for the gas operation, and the switch without lights would be for the electric mode. Prior to operating either switch, make sure the water tank is full of water. Turn your hot water tap on, if water comes out, tank is full. Connect trailer to 110 Volt source, turn circuit breakers on, Place switch without lights to on position. If water does not heat in 45 minutes, Install new heat proble. The gas mode should be repaired by someone that is familiar with that system. Atwood can provide you with a local service center.

It usually only lights when starting up on propane, or if it didn't ignite for one reason or another. Usually light off indicates working o.k. Are you sure the electric element hasn't been on and water is heated already? Carefully open pressure relief valve slightly and see if water is hot or cold.

Have you checked for additional switch, perhaps on rear of water heater on side of cover that wiring goes into for element. How far have you gone with it?