easiest way is to mimic your RV's system. do you have a 15-amp (1800-watts), 20-amp (2400-watts), 30-amp (3600-watts) or 50-amp (6000-watts) system? amps x voltage = watts. this assumes nominal voltage of 129VAC. but you will likely never draw more than ~80% of available amps. and a 50-amp RV system is actually two legs of 50-amps or 12,000-watts (2-legs of 35-amps & 8400-watts). if you plan to use your A/C be sure to account for the ramp up voltage.
otherwise add up the required voltage of whatever you plan to use (assume simultaneous use for a worst case number), add 20% and go from there.