[ecomodded]

2 deep cycle batteries will give you a lot of juice/ drive time.
Two 80 ah battery could provide 8 amps for 20 hrs. drive time,20 hrs by lets say 50 mph is 1000 miles a charge. Adjust accordingly for your individually cars amperage use.
Solar may come to mind for many of us when we think of ways to recharge the system, but i think the real cure is wind power.
Its something anyone can make. Made mostly from everyday items, with only a few odd pieces like couplers, a charge controller etc.
A car alternator would work for a wind generator, but is not ideal as it is a high rpm motor.
If you can find a big old heavy motor with 700 to 2000 rpm stamped on it you are in business.
I have been looking into wind power as of late and can really see the cost per watt benefit of wind over solar. Solar is expensive.
That being said i have a 15watt panel on my dash board that keeps my battery at 14 volts.