I think they should have sold him the part he wanted, cash. Saturday is a prime day to do maintenance on RV's. Anyone that assails a customer is a fool, regardless of whether they are closed or not. I would return to that dealership on their next open day and have a talk with the manager.
I walked into my bank on MLK day, I forgot about the holiday, they had left the doors unlocked. Two cops and a manager greeted me about half way through the lobby. They did not holler at me or threaten me or try to belittle me. They calmly explained the bank was closed and apologized for the open doors. Why the cops were at the bank I have no idea, don't care. I left the building, they locked the doors episode over. It could have been a serious situation.