ummm,
Craigslist?
It would really help to know where you might plan on parking it. Then if you have a special lake or campground that you are looking into, consider asking if any of the RV's already set up might be for sale.
Medium park model leaves a lot of open questions.
Do you want a towable model? What you might tow it with?
How old is acceptable to you?
If you want a "Hunting cabin" it might be a few years older than one you would find acceptable to the wife and kids to go camping with friends.
If you want to be on a lake, you probably can find several places that have a truck to move around the 25 - 40' RV's from a storage yard to the camp site a few hours before you arrive. Then they can move it back to the storage area on Monday or Tuesday.
Usually there is a place that has small mobile homes and RV's with a deck already built in. You can find them in your part of the country.
Fred.