They're probably telling you the truth, too. I mean, if, say, your unit is worth $8000, then they probably can't justify giving you more than $6000 for it, because they have to do their usual dealerish sprucing up, etc., and then they need to make a profit to cover the their for-profit business expenses.
Since you're probably as likely to be able to sell said unit for $8000 yourself, you'll make the extra $2000. Since selling a clean, well-priced fiberglass trailer is not too difficult, there's your extra money.
Granted, that's true for ANY item you trade in, generally (that you get more for selling it yourself), so it probably does indicate some reluctance for the dealer to take on something unusual for them. In other words, dealers would normally suppress that information when a unit is something they do want to take in and sell, and they were simply making an exception with you and telling you what holds for all units. There were probably other factors at play too, of course.
PS: What's "NFGR" stand for?