I sold a 2001 Casita 16' Spirit Deluxe in 2012. I sold it for 3/4 of what I had paid for it (very slightly used) 11 years earlier.
What helped me in setting the price was looking at what other Casitas of similar age etc. had sold for here:
Casita Trailers SOLD | Fiberglass RV's For Sale
It's not always clear that the price shown in the "Sold" section of this website is the actual sale price or what the original asking price was, but you get an idea of what the market value is for your unit.
Our Casita sold at full asking price within about 10 days.