There isn't much available in propane 12v
generators. Let's take a look at what's been available, now and then.
1) In the 1960s Airstream had a propane 12v
generator as an option. There are some surviving examples, but it was loud, unreliable, and had low output current. I believe it installed in one of the baggage compartments.
2) As noted upthread the battery charging outputs of various 120v generators could be used. These use gasoline engines, but propane conversions are readily available. In general, again as noted upthread, the power output is extremely modest.
3) It is possible to assemble something using an automotive alternator and a gasoline engine (converted to propane) connected by a belt. The belt drive is necessary because automotive alternators require a fairly high shaft speed to produce much output.
4) There are some commercially available 12v
generators that appear to be made up of an automotive alternator, a small gasoline engine, and a gear drive. This reduces size compared to a belt. Prices start at over $1,000. http://www.altendc.com/g1-gasoline-b...g-dc-generator
5) In the recent past there were permanent-magnet 12vdc generators available that used a large diameter, balanced rotor that was bolted to the crankshaft. They were mainly sold for marine use. I can't find any for sale now. By memory, they were fairly reasonably priced and had output of around 50-100 amps.
All of these are large. PV panels have come down in price so much that, today, I'd opt for solar panels first.