Just a few key points in the dealers defense...
When a customer does not provide a VIN then millions of dollars worth of research, record keeping, open recalls and other vital data about that vehicle goes right out of the frigin' window. You have just asked that dealer to ignore all of that documented data and to "fly by night".
A good way to get ones butt in a sling.
That dealer probably sees one CTD customer at the parts counter a week, the balance of the business is cars and gas pick-ups. Of course the parts staff is not going to know what your truck takes for filters right off the top of their heads. They can probably go grab an air filter for a Caravan from memory because they are working with those all of the time. A fuel filter for a CTD, they need a VIN and rightfully so.
If you own a Sterling, Western Star or Freightliner product and come to my parts counter without a VIN and ask for anything much more than a chrome nut cover you will leave empty-handed. I will not expose my company to potential liability because a customer says "They are all the same....."..........
Mike.