Here is a starting point:
https://www.nadaguides.com/RVs
You can also compare local Craigslist sales of similar trailers.
You can also get an idea in
https://www.rvtrader.com
The bottom line is it is worth what you can get someone to pay for it. They usually sell better in the spring, but sometimes life doesn't go that well.
Remember dealerships have advantages in selling sometimes. Not everyone can pop out $13-15K for a trailer or get their own financing.
I had the advantage of military sales lots on the 2 that I have sold.