It is a guess, but it sounds like you may have blown a fuse for the circuit which powers that section of the Argosy. I don't think any of that era originally had electronic spark ignition water heaters, so yours is a replacement unit. Whoever put it in may have taped onto that same rear circuit for the power to run the electronics in the DSI water heater, and without the 12 volts, it will not run. So, see if you can find the problem with that circuit, and there is a good chance the DSI water heater will run on gas.
BTW the 120 volt electric heater you apparently have in the water heater is a completely different system, with no connection to the gas side of the unit. You need to sort out some 12 volt
problems I think.