Crockett has a good point. You may already be informed about this, but you can get a Suburban water heater with gas firing and a pilot light, or with gas firing and direct spark ignition (required 12 volts) or gas direct spark plus a 110 volt heating element.
The direct spark doesn't require much 12 volt power, so you probably wouldn't notice the drain on your battery.
The key question is, how will you camp? If you seldom have hookups, I'd recommend gas only with direct spark ignition. It only costs about $30 or $40 more, but you'd need to run some wires. 110 volt element adds about $100 more.
2011 Sunseeker 3100
1997 Pathfinder Toad