> I haven't had a credit card of any kind for at least 15 years.


Bingo!

Your score is probably low because without any credit cards, you have no credit card payment record. This, in their convoluted thinking, adds uncertainty, as you don't show a record of on-time monthly payments.

If I were you, I'd push the dealer and suggest purchasing the vehicle hinges on getting a good finance rate. Sometimes the dealers have some influence and can push things through... but only a limited number of times. So they often don't "go to bat" for people unless they really think the sale hinges on it.