By "lot rot", he is referring to the fact that up till this point nobody has wanted to buy these units. It could be because they are less desirable than the competing units, or they were overpriced from the start. No matter. Look at them to see if any will work for you.
Drive a HARD bargain. Forget about building a relationship with the dealer... A dealer with that many old units is bleeding money badly. They may well be about to close the doors because of it.
A deal like this is all about the money today.
The deal they have offered so far, is not a deal at all. You can get 35% off MSRP on current units if you try hard.
If and only if I liked the unit, I would offer 60% off cash, or walk away. If you are needing to finance or do a trade in... That muddies the water. That's why I don't do it.
Many manufacturers require that the roof be inspected and recaulked every 6 months.. I doubt that this has been done. Make sure that they do this for you before delivery... Get it in writing tat it has been done.