I called another propane company and turns out they were the ones that used to fill this tank for the homeowner that used the pool & spa, 6 years ago. They came a few days ago and checked it all and replaced the regulator and looked at the heater and lines and deemed it all fine. Today they came and filled the tank. It is 125 gallon.
I was thrilled to hear the tank and heater were fine, in his opinion. He said these older Raypaks work forever. But I think I do need a heater service call because it fires up and puts out some hot water for a while but then it stops, like its thermostat is off or something. Or maybe it's flow problems or a dirty filter or something. The 'call for heat' light is lit, so maybe it's not the thermostat? Or maybe that light is on whenever the heater is on? I don't know. I also don't understand why when it's not putting out hot water, the heater is heating (or there is heat radiating from it), and the gas lines are hissing like gas is flowing, so how could the water possibly not be heating?
The propane serviceman suggested I set all the suctions and returns to the spa when I was heating it. I'm pretty sure the spa skimmer line is clogged so I guess it's sucking entirely from the spa floor drain?