If you want a rv genset look at craigs list. they are usually there all the time. But as with anything used it will need a REAl good checkout, not just a start and run. I have seen them for around $200-400 dollars but like I said I dont know their overall condition.
Also look real close around the AC unit on the inside, there is an article in the recent MH/trailer life about how the seals go bad and leak there so you would need to pull the cover off on the inside to check. A friend had a 5th wheel and it was flooded by the rains comng in around the skylights and vents so check all opening. Pull the panels off the sides and check under the refrig, the water tank etc or atleast have a knowledgeable person do it with/for you.
A lot of good buys can be found on CL, but you need to expand your search area at the same time. Before we got the one we have I was looking at texas and calif sales and there were some reasonable one, but the newer ones you would think were on a dealers lot. SOme were a lot were not, but there are deals to be found IF ya look. Thats where I found ours, about 250 miles away only a year old with 4000 miles. Same one we almost bought at a dealer for a lot less.