Especially good deals are to be had on giant Class A's (over 33' long) because it's so much trouble to keep them if the house gets repo'd. Most apartment complexes won't let you park a vehicle that can't fit into a single assigned spot. If it's longer than an 18-passenger extended 3500 van, it's probably not welcome.
Finding a driveway to rent (or a helpful friend with unused long driveway) is tough.
If you park on the street in Portland, even with current tags/clean/respectable-looking, expect a green tow-warning pretty soon. If you live in an RV, parked on the street, expect continuous hassles with short breaks, if you leave and return, when they find you again.
When I say "really cheap", I mean it. This was 5 years of almost-a-house-payment in 1982, now 3 weeks of full-time pulling coffee.
Of course, it's you-zzzzz-d. The trick is to buy as good as possible for your money (recently replaced engine, transmission, new tires, new battery, now fridge, everything works, etc.) but "fully-functional" and not dripping fluids is realistic. Blue tarp is actually not an indication of fuzzy-mold wet-disaster, but instead a realistic owner who has preserved the old gal for you. Don't really expect that someone has last-year dropped $6K into repairs/resto with 300 hours labor and now will sell you the rig for $1500, but crazier things have happened.
Some people have a problem with 1970's "rustic cabin" styling in vinyl sheet and foam. Don't worry, half-a-day with a pry bar and 3 pound hammer can have it ejected out the door. My trailer is blessed with "foam colonial".
Why would anyone want to even get one of these beasts FOR FREE? Because a 33' RV is a 250 square foot apartment on wheels, with the ability to support modern convenient life for as long as the fuel/power holds out (a week at Burning Man), remote from plugs, hoses, drains, etc. that are the current civilization. If you have the space to park it safely, it's an insurance package against disaster coming to your neighborhood.
If you have a big V-8+ truck, the same deals in 5th wheel trailers are out there. Same difficulties with parking, but no drivetrain to worry about.