Unless that Atwood is an XT Model...there is no 'mixing valve'. Only the XT Models use one.
IF it is an XT Model.....mixing valves do have problems and have to be replaced
SO what is the Model of that Atwood?
a 'GE16XT' (10 gallon)
OR
a 'GC10A' (10 gallon)
How many valves on back of WH?
3 valves cold/hot OPEN........middle (BYPASS) CLOSED
2 (3 way) valves...this one is in BYPASS (handles are parallel to middle line). Would need to turn both handles parallel with cold/hot lines to be in service
2 valve (cold inlet, BYPASS with check valve in hot outlet) Cold OPEN BYPASS CLOSED
1 (3 way) valve. Cold inlet-----handle parallel with cold line (BYPASS---handle would be parallel with middle line) Check valve in hot out
Need to make sure that any BYPASS valve is CLOSED. Otherwise cold water will mix with hot water making 'warm' water
Also check shower(s) inside/outside that they are turned off with cold/hot knobs and not just the shutoff on shower handle.
Cold/Hot will mix inside shower faucet giving you warm water
