Don't just contact TT to confirm. READ your contract. My wife and I got into Thousand Trails as a part of some membership she was sure she wanted. We never used it, so after 3-4 years I told them I wanted to drop it. I got the same answer as you -- you can't.
I read my contract and it stated that I could drop it anytime I wanted to. They insisted it wasn't so. I copied the contract and emailed it to them with parts of it highlighted. After a few days they said they'd spoken with their legal team and I was right.
Just check it out before you take their word for it. If you don't have your copy, ask them to send you a copy of theirs, the one with your signature on it.
The story I got was that they'd changed their contracts shortly after we signed up. Who knows?
Good luck!