I have a model GC10A-4E. Worked perfect until main water supply was shut off outside. I did not know water was

Hi my name is Robert...
Few things. First on the back of the tank you should have some valves, a hot cold and bypass the hot and cold should be open or valve handle running with the pipe and the bypass should be shut, valve handle running against the pipe, sometimes you have to go inside the RV and pull a false panel or drawer to get at the back of the water heater.
Next if you have an outdoor sprayer make sure the valves are OFF, I have seen some strange things happen when they are left open.
Finally the most probable cause is a failed check valve. When they fail the spring and disc can fall out and block the flow of hot water, the check valve screws right into the port on the back side of the water heater, it will be plastic or brass and have tiny arrows on it for direction of flow. It will be about 3 to 4" long and just looks like a normal straight fitting but it is a check valve. You could remove it after draining the water and see if it works without it...
Hope this helps some, IF you have more questions or concerns please feel free to contact me... Robert

OK
Normally the XT model only has the mixing valve and it will be on the back of the tank. Atwood says that they are no adjustable but I can tell you how to work around that. You should have a Red or yellow knob, and there may be glue on top of the torx head that holds that handle on. After the knob is off you should see 2 sets of splines, the small one is to adjust the temperature and the larger one is to lock the knob to prevent it from being adjusted. Might need to make several adjustments a little at a time.
That only applies to the XT model, if it is not an XT model then I would be looking for the check valve on the back of the tank... Robert