I have run my deuce at "low" (1,000-1,500 ) revs for hundreds of hours over the years, operating the crane. The engine needs only 1/2 hour or so on the highway to clear.
It uses about 1 gal/hr, if not loaded down at those rpms.
Like WM said, you need deep cycle batteries if you plan to run them down, "starting" batteries will not take a full charge after only a few deep cycles.
To shorten the charge time, use NiCd (aircraft) batteries. They can handle a thousand or more deep cycles, but will require at least a 100 amp generator/alternator...(edit) unless there is current limiting circuitry in line...