Question #1: Why at no point did you ask to speak with a manager?
As a customer not knowing ERAC's operations, most would assume anyone working for a rental company must be pathetic. So why would you trust, what would appear to any normal person to be, a minimum wage employee to check for you and then just accept the half-hearted response (which you acknowledge) if these were the most important valuable items to you?

I would be damn sure I got a definitive NO! You invested a lot of effort calling various organizations, but spent all of 2 min and little or no effort further investigating something you are SURE was there.

I empathize with you, but like the first responded said, it is 100% your responsibility. Should the agent have checked? Of course. Is she lazy and incompetent? Most likely.
But you just gave up. What does that make you?

Maybe I just know the system too well, but when the second girl said the vehicle had left the branch, wouldn't the smart thing to do have been to ask where it went, what customer was in it? Was there any way they could be so kind as to call the customer to "double-check" because these items meant the world to you.

Did Enterprise at various steps along the chain fail you? Absolutely. And I think it's very important that you do send the letter, but between you and me, it will make as much difference as this website has in changing ERAC's operations - NOTHING! Consider yourself lucky if they even respond to you never mind give you an apology.

The reason for the existence of this site is the same reason you didn't get your wallet back. Glance at any of the tens of thousands of postings on this site. Now multiple that by every branch in America. If you think it's different in other parts, you're mistaken. This company is fucked from the top down.
If you hated everything to do with your job, were making a poverty level income, worked 57 hours a week, and had no time to interview for another job, and therefore no hope for the immediate future, how good a job would you do at work? How much would you "care"?
You could tell people to just quit and find a new job, but it's not necessarily that easy. Between having no time and no idea what they would do otherwise, combined with the worry of not knowing how they were going to pay their bills if they quit, how much hope do you think they have?

You threw your wallet to the wolves.