Your best bet is the archive of past sales at Molded Fiberglass Travel Trailers For Sale | Fiberglass RV's For Sale
. There are so many variables of condition, location, season, and equipment that any estimate based solely on the year would necessarily be a broad range. I have seem them offered at prices ranging from around $5K to over $12K. The year of manufacture is a lesser variable, so I would look at units with similar equipment in a range of years around 2008. Pay attention to where they were sold (prices near major population centers on the East and West coasts are higher) and the time of year (spring & summer prices are higher).
If you have a link to the actual trailer you are looking at, you'll probably get some more specific thoughts about its value.