In '79 when my bus was converted Onan was about it. My 6.5NH still works like a champ. By them on craigslist all day long for $400 to $800. The 6.5 will run two airs and a few other things. Many bad mouth them and have never seen one. For the money you get a good system. As with any genny, just make sure it runs before you buy.
The Onans are also easy to convert to propane. Many of them come set up for "tri-fuel" (gasoline, LP or natural gas) just by adjusting the carb.