Hi,
check the manufacturer's specifications for Amperage draw on start up. I installed a diesel Airtronic heater for cold nights, and was a bit surprised at the battery use.
Mine uses a glow plug drawing 8Amps for a while at the start of every heating cycle - not sure what the water heaters do, but if you are aware of the potential problem you may be able to plan a way around it.
Have you looked at a gas option?
They start with an electronic spark.
Much less current needed, but on the other hand more gas is needed than diesel for the same amount of heat, and gas in bottles is more difficult to ration than diesel in a tank.
cheers