Should I offer less for this new Hail damaged KZ trailer?

I would walk away, that's entirely too much damage. Also, aluminum roof is a little strange as most TT have a rubber or TPO roof. Better RVs may have a fiberglass roof.
Also, after purchase of a new unit, you would want a warranty on the roof. Much too easy for leaks to occur and you would have no recourse. One small leak can cause major delamination to sidewalls.