I bought a 20 year old Kirby 3G for $30 on Craigslist.
The 3G model is very similar to the ones that you can buy brand new. They've change the colors and very little else over the years.
They usually go for between $100-$200 used (and $1800 new), but there are a few sellers who will sell it for almost nothing simply because the belt is broken. Replacing the belt is as easy as replacing a light bulb, but some people are mechanically challenged.
Admittedly, the Kirby is overkill for the motorhome, but it does do an outstanding job.
I've had too many cheap vaccuums break on me, so I'd rather buy a quality used one.