As I understand it, LP is about 10% less efficient than gasoline. A 20 pound fiberglass LP tank would be ideal, and probably comparable in range to 3 and maybe 6 gallons of gasoline. You could also carry an adapter to use 1 pound canisters as "emergency" get-home backup or "convenience tanks" for short-range. You can buy those at many marinas and most hardware stores/departments, even supermarkets.