Unwanted Hot Water

I had to replace all the fresh water piping in my 1982 Coachmen RV. I replaced the cold lines and then the hot lines so that there were no mistakes. But when I turn on the cold water in the bathroom I get hot water for a short time, then it gets cold. What's up?
Peter, (Nesquehoning, PA)

Peter, due to the layout of some floor plans, it's not uncommon for a cold branch of the fresh water system to be physically connected very close to the cold inlet on the water heater. And without a definitive method of keeping the heated water from backing into the system tubing, it's inevitable that some of the water in the cold line at the inlet will also be heated. During the time your coach was manufactured, it was rare to find a backflow preventer at the water heater, such as common today. If you are sure you have indeed replaced the tubing properly, try this; Install a one-way check valve (backflow preventer), directly to the back of the water heater at the cold, input line. Keep in mind the direction of flow. It should be positioned so that water will flow into the water heater but not out. My guess is that some of the heated water at that inlet area is being siphoned out of the water heater and delivered to that one faucet when it's opened.

To test this theory, fire up the water heater and open the kitchen hot faucet making sure hot water is indeed flowing. Then turn that hot faucet off and open the cold water at the questionable faucet. Chances are the problem disappeared. Why? Because as the hot water was running in the kitchen, cold water was automatically entering the heater eliminating that small portion of heated water near the cold inlet and that branch leading to the bathroom. Installing the backflow preventer should eliminate this condition by prohibiting any heated water inside the tank from migrating back into the cold line those few, crucial inches.