I found a great deal on mine -- notably less than others I had seen online -- and paid very close to asking price. There was certainly no 20% discount, and not even 10%. But I also had to drive 2,500 miles to pick it up.
Your adjuster's comment may be quasi-accurate for dealer sales, especially for new units, but certainly not as a rule for private parties. And even some dealers may not have "used car lot pricing". Look at CarMax....I realize they don't sell TMs, but try to go in there and pay 20% less than sticker, and you will find yourself frustrated.
Since he seems to be so knowledgeable about buying trailers, why don't you ask your adjuster to find you one?
All you want is "to be made whole", i.e. you simply want to get back to the point before the damage. So you might tell him if he could just buy you a 2003 3023 in comparable condition within 100 miles of your house, you'll be happy.
Dave