RV Repair: Rv water heater, venturi tube, pilot lamp

Question
I have a water heater in an RV that I own, when I light the pilot lamp, it burns fine, and will stay lit, the thermocouple I believe is working fine, then when I go to the "run" position, it seems to light the main burner for a moment, then it goes out and the flame jumps back to the venturi opening where the air mix adjuster. I have removed the venturi tube from the gas valve, and found some loose "crap" insde, and retried many times. I have been told that low propane volume could be the cause, but this is now happening on a friends trailer as well. It worked 2 weeks before and I believe that there might be some dirt simply plugging the venturi opening. What do you think? Finding a leak over the vast piping in a 35 foot trailer may not be simple. Are all the lines steel? Could they be rusty and plugging?

Answer
Hi Bibll:
Sounds to me that the line from the tank to the water heater valve is restricted and that the line pressurizes past the restriction when you shut the valve off. When you start it the gas volume starts the the burner but can't maintain the flow. The line is steel except near the tank and then going to the valve. I would take the line loose from the tank and the valve and then blow it out with pressurized air. The gas pressure is only 5-7 psi so the air should blow out any restriction. Good luck.
Bill