When we camp, the battery has never been enough. Even in my more 12V efficient old trailer.
So, considering gasoline has about 300 times the energy density of lead-acid batteries (45,700,000 vs. 146,000 Joules per kilogram), I decided to go a different route to have enough 12VDC power when we boondock.
I built a 12VDC genset from a new ($100 Harbor Freight engine) and other bits and pieces:
The instructions for building one of theses is all over teh intarwebs. http://www.metalwebnews.com/howto/ge...-generator.pdf
Connecting it to the RV is more fun:
I used 175A genderless DC connectors (similar to a previous project).
The little bugger stows easily:
So one 2 gallon can of gas (about 6.3 kilos) has the equivalent energy of about 1,900 kilos (literally, a couple of tons) of lead-acid batteries.
Even considering the HUGE INEFFICIENCIES of converting gasoline to electricity, I'm still ahead of the game in the amount of weight carried vs. the amount of electricity I'll have at my disposal when camping.
Couple this little guy with the RV's battery and a 2,000Watt inverter (from that other project) and I can run my coffee grinder as much as I want.