Towing new trailer home with out WD

Your dealer is an idiot and is trying to force it so you have to buy his hitch or at least pay for install. I understand possible liability issues if you hurt yourself doing the install on their property, but what about the liability when he forces you to leave with an unsafe setup?

You could overstress the receiver and cause failure when you greatly exceed the rating. If they refuse they should at least be willing to deliver for a minimal fee.