The pilot light lights fine with a nice blue flame, but as soon as I let up on the pilot button the flame goes out. I've tried a new thermocouple - two of them, tried disconnecting the pilot fuel line and blowing it out with an air compressor, tried taking out the burner assembly and blowing it out - all with no luck. The H2o tank is connected to 100 lb tank along with a fridge and stove, both of which work fine. I've tried turning off the other appliances and just lighting the H2o tank, and still no luck. Any other suggestions out there?

You need to test the solenoid coil in the gas valve that holds the gas valve open for the pilot light. Use an ohm meter and check between the center terminal where the thermocouple screws in and the gas valve body. You should read several kilo-ohms, but not infinity. Also, check that the flame is hitting the 3/8" part of tip only. If it's not hitting the tip just right it will not generate enough electricity to energize the solenoid enough to raise the pin from the diaphragm in the gas valve which opens the gas valve and allows the main valve to open. My guess is that if the flame is right you need a gas valve. It's really best to check the gas valve before just guessing at it and putting in a thermocouple, although most of the time it's the thermocouple that goes bad. There are little testers to test thermocouples that you can buy at any HVAC or plumbing parts store, but you need a meter to test the gas valve. Some meters will test millivolts and therefore can test thermocouples.