If you have an outside shower or faucet, check that it's not on with both cold and hot water knobs, and the hand sprinkler being the only valve holding the water off.
Cheap fix, and something we all overlook at one point and time. It mixes the water and will cause the problem you have.
Lets cross our fingers and hope this is all to your problem.
Dan & Loretta, US Army Retired Aero Scout Pilot
2012 Fleetwood Providence 42P Class A/DP
Spartan Chassis, 8.9L Cummins 450HP