If your goal is to do this as affordable as possible, you'll want to sell your trailer and truck by yourself, individually. If your trailer isn't worth much, you might want to offer it up as part of the deal for the truck advertising it on Craigslist that way..."Buy This Truck And Get This Travel Trailer FREE".
If you don't want to be selling them on your own, then trading them into an RV dealer is the way to go, but you won't be getting street value.
I know that
Phoenix USA takes in just about anything as a trade-in. They will deliver your new motor home to your front door and drive away in your trade-in. They'll give you a price on your trade-in after some pics and data are reviewed. Order on-line, stay at home, and let someone else do all the footwork. It's not the way I would buy a motor home, but many people have done it just like that.