Is this a sDealer that you will be 'working with' in the future? ie. is he the main person that you will be taking your unit to have work done on it? If he is.... he can screw you over royal and 'get his money back' for any future work. If you are close by to his Dealership, sit down and 'talk' to him. He has 'lost' nothing .... he just did not 'gain' as much as he thought that he would have on the resell of your/his unit. If you truly did not know of the 'bad' roof, convey that and let him understand (nicely) that the trade in was 'as is'. He can 'fix' the roof, he just does not want to put out any money to do it. Do not be talked into 'rebating' him, if you are in the right. IF you will Never see this sDealer again... you have no obligation to 'help' him reduce his loss, but know that if there is any warranty work to do.... He can and most likely will make you regret it if you take your unit back to him to work on.
(Do I know this for a 'fact'.... I know human nature.....)
2013 Forest River Salem Hemisphere SBT312QBUD
2001 CrewCab F-250 7.3 PowerStroke Diesel
SuperChip, BTS transmission, 6.0 Trans Cooler