Is this by chance the Predator 3200/4000 from Harbor Freight that you are looking at? If so pass it by. It is really like two 15A 1600w generators. It won't give you 30a, or even 20a, at 120v.
The Champion 4000 can be had for the same price and has a 30a RV outlet right on it. Much better choice in my opinion.
Both of those are open frame models, the Champion being the "quieter" of the two. You will have to spend much more for something quiet (inverter). The Champion 3100 would be my choice there. It has been on sale several times at Cabela's for $699.
Mike