Looks interesting.
I don't know about this one in particular, but fuel cells are basically chemically converting the energy from the methane (CH4) into heat, water and carbon dioxide - while generating a voltage potential (DC power). So, you can charge your battery, then use your on-board inverter to power the RV. Not sure how many amps you could get out of something like this... The biggest unit shows 210Ah/day (or 8.75Amps average)
How much you could use at any given time would probably depend on your battery/inverter system. Looks like their largest system is rated for 105W maximum. That won't run much by itself... but it will eventually recharge your batteries.
-Tina