If the dealer valued your trailer as if it was just going to auction then they gave you the lowest trade in value for it. They are not going to ask for more money. If they get more than that it is good for them. Obviously you are happy so I wouldn't worry.
A few years back we made a deal on a new truck. The trade in $ we were allowed was quite generous we thought, considering the trade in would be going to auction. The day we drove trade in for the new one it had a major malfunction on the 2hr drive up. It had to be towed. When we heard all that was wrong we felt guilty and offered the dealer to re negotiate the trade in $ since he was not getting what he thought he was. He said no, the deal stood. There are still some dealerships that go above and beyond. They even paid the towing as they considered the truck theirs from the day we made the deal.