I'm glad to hear you had a great, memorable trip in your trailer. Making those memories is what RVing is all about.
Without seeing pictures, it's pretty hard to assess the condition and value. I can safely say that with it sitting since 2015, it's very likely the trailer will need new tires. Unfortunately trailer tires only have about a five year life span, and not replacing them can cause blow outs, which will tear up the outside of the trailer and cause a lot more damage. Sunline plastic parts and metal to match are not impossible to find, but they are a little more unusual and it would cost a lot more to have blowout damage repaired than to just buy new tires.
The other fear is water damage. Trailer roofs need to be inspected at least yearly and resealed at the first sign of any sealant failure. Otherwise water can intrude and rot away the wood inside the wall, which you won't discover until the damage has become pretty severe. If you don't know about the history of the roof's service, it might be a good idea to have someone check it out.
On that note, whoever you have look at it needs to know that the roof IS NOT walk-on. They can walk on it by placing a blanket or towel on the roof and then putting plywood on top of that to span the rafters. The roof itself is not self supporting.
Do you have any pictures available of the trailer?
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR