Bill is right about the AquaHot not being as expensive as it sounds, given the overall picture. Forgive me if Jerry's number of .03g/hr sounds not quite right, though his experience appears to back it up. Perhaps in warm ambient conditions with the electric element or residual engine heat assisting, but burning less than 4 oz (half a cup) in an hour under normal hot water use and even summer night time ambient conditions sounds implausible. He must have really really great extra factory insulation.
The point is that the burner runs on demand. The demand can vary all over the place, with number of people in the coach, outside ambient temps. anywhere from full summer in Phoenix to dead of winter in Saskatoon or Fairbanks, warm nights or cold, windy or no wind, mountain or sea level, whether you use an auxilliary heater inside, whether you are plugged-in or not or use the electric element, whether you use the vents a lot or crack a window, whether you use paper plates or wash dishes, whether you shower daily or once a week, and of course where you set the thermostats or if you use the heat pumps. A half gallon per hour is a number I've heard many times from many sources over the 6 years we've had our diesel coach, and I take it for what it is... a ball-park answer to a very general and unqualified question.

Joel