I have turned on my Dometic on 12v and it'll go about a day before it
drains the small starter-sized battery.....but mine happens to work quite
well on propane, which I have in a normal tank under the sink, with vent
holes drilled through the floor (propane is heavier than air) in case it
springs a leak...a 'regular' tank last me about 3 weeks.  I leave it on all
the time when I am camping or traveling or at a bike race...Much easier to
fill, you don't have to take the whole van to a hose...