Sadly that is how National rolls 99.5% of time; they promise to get you "a" vehicle, but not necessarily what was reserved or wanted. Yes, rates will be adjusted as required for any modifications, but other than that they consider themselves having upheld their end of contractual agreement.

Find only way to possibly be part of that .5% that does have success is where location (especially manager) knows renter (frequent flyer so to speak), and thus out of courtesy may pull something and put it aside (subject to availability), but if inventory is tight all bets are off.

Depending upon how much of a stink you're willing to make could always reach out to EC CS to see if they will give you something for your troubles.