Hi gramcrackers, and welcome to the forum and Heartland family. While I don't know if I can help you directly a few thoughts come to mind. If the water heater were to run continuously, then I think that you would blow the high pressure relief valve. Is that happening? And, much propane are you going through? Depending on the outside temp and what the thermostat is set at while using the furnace and LP for the water heater, I can go through a thirty pounder in three or four days.
I try to use electric for everything whenever I can.
Peace
Dave