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

Most likely the dealer has already had his insurance company cut his business a check to make the repairs. If he can sell the trailer at his "discount" price as is he gets the repair money free and clear.
You can offer him $10,000 for the damaged unit knowing you will be spending more in the future to repair the damage or, buy elsewhere, save money and avoid this dealership in the future.