I believe the RV folks use a second battery with an isolator connection, which allows the second battery to charge along with the primary, but allows the second battery to discharge independently. So perhaps a second battery would be a better bet. Far as that goes, lead-acid is far from the most space/weight efficient when it comes to charge capacity. I'd probably go for NiMH or Li ion as a secondary battery. As suggested above, try push-starting the beast (especially when cold) and perhaps you'll be convinced that running the battery dead is not a trivial matter.