Here in New York there is no difference between fleets of Alamo and National. That is because several mergers or whatever ago state forced company to combine both at same locations. So any National location is also Alamo; thus lots/garages share same fleet. Differences come down to rates, rewards and so forth.

Enterprise OTOH is an entirely different beast. Yes, their vehicles are often older, and sometimes not in the best condition. Not quite at "busted up" stage, but close enough in some cases.

Maybe because of their high volume of loaner business where a third party is paying (thus the real customer) Enterprise has a different view about fleet composition than say Enterprise.

Not mentioned by OP, but never the less unlike National Enterprise doesn't give up anything. If you reserved economy or intermediate and all they have is some near busted wreck, that is what you'll get. They will happily give an upgrade *if* you are willing to pay extra. Meanwhile National in my experience if fleet inventory allows will often give the upgrade at same rate/free of extra charges.