They're all about the same. The most important thing is to buy enough power. Unfortunately the gensets are rated in watts while you are concerned with amperage. Generally the watt:amp ratio is about 10:1, so to get 20 amps all the time without destroying the genset you would probably need 1800 to 2000 watts.
The reason I say that is because, once plugged in, everything in your RV will be using that power - fridge (unless you switch to gas only, but even then the lighter will use some) A/C and 12V invertor that powers the lights and recharges the battery.
The Yamaha I listed from generators direct is 900 watts, but still has a 20 amp circuit. You should probably look at one or even two levels up from there.