Dan,
Why not an inverter? They're available in several sizes and although you'd need a separate battery, they can be connected together using a continuous duty Ford starter solenoid so both batteries are charged simultaneously when the key is on and motor is running, yet isolated when the key is off. I use this setup in my service van and have a 4000 watt (peak, 2000 continuous) inverter ready and willing whenever needed. When the rear battery gets drained, simply start the vehicle and continue to use the inverter while it's running. This setup keeps the rear battery charged while you're driving and allows you to conserve gasoline while not driving and still have electricity for powering stuff. Been using it this way for over 10 years and works just fine. With today's smaller batteries, it would be easy to mount the battery under a seat or in a corner and have the inverter conveniently located anywhere you needed it. No need to use a deep cycle battery or anything like that, just make sure that it's well fused (Maximum 200 amps). If the rear battery loses it's ability to hold a charge, swap it out...most car batteries today have a 3 year free replacement anyway.
Warren