No Output - RV Water Heater

I am having problems with the hot water heater which is both gas and electric. The tank fills up and the water gets hot but when I turn on the hot water at any faucet all I get is cold water. I checked the valve and it is open on the water heater. There are three valves; one is the cold water feed and one is for hot water and there is another pipe the runs in-between the cold water pipe and the hot water pipe. When I turn this valve off I get hot water for a second or two and then I lose all pressure and nothing comes out. But when I turn this valve back on I get water pressure, but only cold water.Any ideas?
Gerald, (Mansfield, OH)

Gerald, this appears to be a problem with those hot water bypass valves. The pipe that runs between the hot and cold water lines is there to put the water heater in bypass mode for winterizing. It allows the antifreeze to cross over from the cold water lines to the hot water lines without filling the water heater tank. This valve must remain closed for normal operation. It is open only for winterizing when the other two valves are closed. Since the hot water outlet is at the top of the water heater, you won’t get any pressure in the hot water line unless the water heater is completely full. You mentioned that the water tank fills up, but I suspect it may not be completely full when you are checking it. Make sure that the valves on the cold inlet and hot outlet are totally open and that the crossover pipe valve is fully closed. The water heater can then fill normally. 

It can take quite a while for the water heater to completely fill up. You’ll know it’s full if you have water emitting from the pressure and temperature relief valve on the front of the water heater. Once water flows freely from the P&T valve, begin opening the hot faucets one at a time. It will take a few minutes to purge all the air out of the lines. Once you have water flowing freely from every hot faucet, light the water heater or turn on the heating element. After the water heater cycles off, you should be able to have hot water at each hot faucet as soon as the cold water clears. This could take a few minutes of running the hot faucet. If you still do not have hot water flowing through the system, there could be a blockage in the water heater, one of the fittings or kinked tubing coming out of the heater and further troubleshooting will be in order. It’s also possible that center crossover valve is faulty; not truly closed even though the handle indicates it is closed. If you have plastic bypass valves, it might be a good idea to upgrade to the more robust brass valves.