We have the issue. It used to be much worse, very hot to very cold. The dealer replaced the mixer faucet. That lessened the problem considerably. It seems to occur when we are using the pump with water from the tank. When we are on city water the temperature stays fairly constant. The water temperature cycles with the pump turning off and on so it probably has something to do with water pressure, maybe a restriction in a water line or in the faucet itself. I have not noticed the issue in previous trailers we have owned.
2014 Eagle Premier 361REQS
2013 Ford F350 Crew w/ 6.7 diesel