Hi,
Is your waterheater an underseat Carver gas-powered unit with automatic ignition i.e. a separate switch inside the van?
If so these were fragile at best and I would guess that the extra voltage from charging/changing the battery may have tipped it over the edge! Not sure whether the switch or the PCB in the unit will have failed, you will have to try substitution (but check connections and soldering on the PCB first)