You are probably correct. I haven't seen any stats, but it makes sense.
Most used units are bought by people buying their first unit and aren't
sure if they like camping/traveling. They start out as cheap as
possible and work their way up if they enjoy it. Slide-in's are
probably the least expensive for people who already have a truck. Used
pop-up's are probably the next least expensive due to the canvas and
slide problems. Then up from there. In reality, I would assume that the
large Motorhomes have the biggest percentage of depreciation due to the
fact that most people can't afford them, and if you can afford them,
you buy a new one. Bottom line is: Supply and demand. There is more
demand for cheaper units because more people can afford them.

Hank <~~~~~not in demand

snipped

I've noticed the same thing. The price of new ones is nuts
as well.
HD in NY

All I can speak to is the local market for the Casita brand trailer.    I
looked a long time before I found a used one for sale, and it retained a
great deal of it's original price.   Then again, when I sold it (to move up
in size), it retained virtually all of the price I paid for it.

The small self-contained trailers such as Scamp & Casita are quite towable
by smaller SUV's thus much desired by those who want a hard-side trailer
that is fully self contained.