I carry 2 5ltr water containers, i fill at home before going away for a day or two, any longer and a filtration filter sys will be required piped from the main tank.
They drink fuel like a thursty elephant at a waterhole, better a small libery bus conversion, lots of room in mine at just over 22ft, there are lots of later 26ft models about, mine does 30mpg at best, have a look at iveco.
Onr good relay via hi beam triger lever on dash with a on of cut out, then a 2 way rocker to fire the relay from either engine bat or hab batts, so simple to do, no need to get over complicated.