OK, so I'm going to go out on a limb here and say it's not the hot water heater. But maybe the propane tank pigtails. When our 5th wheel was two years old we had an issue with the furnace not running. An old guy camping a few slots down from me suggested swapping out the propane tank pigtails. He gave me an old used one he had and what do ya know the dang thing fired right off. Once I got home I changed both propane tank pigtails out for new ones and haven't had a problem since.
The pigtails have a high flow shut off valve in them that causes the propane flow to shut off in the event of a catastrophic failure. These high flow valves can and will go bad.
