SandraC Maybe I can help you a little, I own an RV Dealership. most dealers who sell new RV's will have used ones as well. Just go to a few and ask for the trade-ins in your price range, if there is none at the time that you like just ask them to call you when one hits the lot. That is often the best way to get a good deal.
When a trade is taken by a dealership they now not only have to pay the bank the cost of the new one they sold but add out of pocket the value of the trade-in to make up the whole invoice price that they owe the bank. What this means is the dealership never see's a profit from the first sale until the trade is sold, therefore they will be more willing to move a "fresh trade" for less money. We do it all the time to keep from spending money on used RV's.
$3500 US should get you about a mid-80's travel trailer 18-22 feet with lots of life left in it.
A few things you should know:
RV refrigerators are big bucks, so make sure it works before you buy it.
Ask the dealer to do a leak-down test on the LP gas system. this only takes a few minutes and could save your life.
Take whatever you buy to a tire shop or service station and have the axle bearings packed, this is a good idea with any used trailer.
Then pull that thing all the way to Disney World and have the time of your life. Hope that helps you. David