RV Repair: SUBURBAN WATER HEATER, suburban water heater, vapor lock

Question
I have a FEMA trailer with a suburban water heater. When I turn the hot water on, the water is hot for only about 2 seconds. After that it is just warm. I have replaced the thermostat and the valve assembly, but that did not fix the problem. Can you give me any advice?

Thanks,
Jim

Answer
are there any leaks?  possible that the PRV is not holding and the water is continuously running?  Is the tank not getting full?  Possible that the tank is not full due to vapor lock?  If all the air did not purge out of the tank the water will only fill some and then new water will rush in and only be luke warm.  Check this by poppin the prv and see what comes out.  be careful, there is burn potential here.