If the unit is pre-wired for a generator, then you're probably better off just having a Genrac 4000 watt genset installed. If it is already set up for it, the soundproofing in the generator compartment will already be there, as well as the wiring and fuel line cutouts.
An installed genset is a LOT easier than hauling around a portable generator and the fuel for it. We had a 2k Honda when we had a trailer, and found that it always seemed to be raining when we needed to use it. And, of course, ya can't use a portable when you're driving. :-)
As with any used RV, have a mechanic check it over.
Good luck and happy camping.