The water heater's power is DC, meaning the igniter, and of course it heats with propane. In your SMB (house) fusebox you should find about a 15 amp fuse for it. I would just pull the fuse. That's only a band-aid but it will turn it off for sure. If yours has a small black toggle switch in the bottom left corner (looking behind the cover from the outside) then it is an AC/DC model and I have no idea, but even then I think it would be on it's own breaker and not linked to the other house ones.
Currently vanless. Weird.