Most RV air conditioners are either 13500 BTU's or 15000 BTU's. A 3500 watt generator is plenty for either of those, RV'ers use that size all the time. The honda EU3000i is very popular (3000 watts) and it'll run most air conditioners.
Wattage Vs. BTU's You might check out:
RV.net Forums Lots of info about this stuff over there.
