You would think there would be a ton of these things, a simple gas powered 12volt charger. What is easy to find is the 120AC/12V generators like the Honda e1000. The e1000 puts out less than 10amps on the 12V DC line. A car's alternator can put out from 50amps to 100+amps. The examples below put out 50 or 55amps, but there are versions that go beyond 100amps charging.
What I am thinking is getting is getting a 12V gas powered charger for my camper. My camper already has a 2000 watt A/C inverter that runs from 4 6V batteries. For extended dry camping, it would be nice to recharge the batteries in a few hours. Using solar is mixed, especially in highly wooded areas. Charging the batteries with my truck would take about 40hours (10amp/hrs for 40 hours for 400amp/hr battery pack).
So, after a search, I found serveral gas powered 12V chargers. The best one that I like seems to be only in Australia. No idea of the cost.
The next one is a chinese knock off that I can get for about $300, but I think available in the US. It has lower noise output than the one above.
There are plans to make your own with a simple lawnmower engine and an alternator.