I'd suggest trying out the onboard generator of various class C's before buying a portable. The installations vary some in their quality. A good Onan installation doesn't have to have a great deal of noise and vibration inside the motorhome, and has some significant advantages over a portable (pushbutton start form the inside, no setup or tear down, a gas tank that's many times as large, much less chance of theft, no need to figure out where and how to store it without getting gas fumes on other stuff....) That's not to say that the portable inverter generators don't have other advantages, of course.
I think you'd save more gas when battery charging (and generator run time) by getting a top-notch converter rather than a portable generator, and also incidentally save some money in the process.
