I dunno, but I think I'd pull that 42 year old water heater out, measure it, and try to find a modern RV propane
heater you can install in its place.
but, looking at that picture of hte plumbing behind the heater, it looks like its been bypassed, and isn't connected at all. the pipes at the bottom are probably the cold water and no longer connect to that T sticking out of the bottom of the heater, while the fitting on the top right might well be the output.
are you getting cold out the hot tap on the sink, or not getting any water at all?
warning, running the heater burner without water in the heater tank likely WILL burn up the heat exchanger tubes inside.
I'd track down all the plumbing and draw a diagram.