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

I think you are on the right track. Just to give you an idea, I bought a TT in 2015. About 2 months later we got golf ball size hail. There are probably 25 dings in the front panel that you don't notice unless you really look for them. The insurance company cut us a check for 50% of the $15000 we paid for it.