My Honda 3000EU will do my trailer. I know it is not rated as high as everyone says you need, but it does the job. I do turn the AC off when my wife is curling and drying her hair, but other than that, I am pleased with it's performance. I will add that I consider it the smallest generator in amperage ratings to do everything. Another option, one of my friends does this, is to get two 2000 watt Honda's and the Parallel Wiring Kit. That gives you 4000 watts of power, generators you can carry by yourself and all the power you need.
Basically, if it is a 30 Amp RV, you should have 30 amps x 120 volts, or 3600 Watts. Since generators are rated at the max power, not the 'normal' power, that means 4000 watts would be ideal.
However, if the generator does not have a 30 amp socket, all that extra power won't do you much good.
So I too, recommend the 3000EU (or 2 2000EUs). They are reliable, efficent and QUIET. I had to install an 'easy start kit' in my AC to get it to start after sitting a while, but other than that had no problems with the 3000EU as long as i didn't use 2 'big' items (AC/Microwave/Vaccum cleaner/Hair dryer) at the same time.