Believe it or not, unless you pre-paid the car reservation (which, if you booked on National's site, you did not because that is not the way the business runs), there is no "guarantee" that what you booked will be there due to ever-changing fleet demands. There's a big movement within the AR industry to change that way of doing business (because of the ill will it generates in situations exactly like yours), but that doesn't really help you with the immediate issue.

So you know for next time, here are a few things you can try if this happens again.

First, you could have requested National find you a minivan with a competitor and had them pay for it. Same concept as when an airline cancels your flight and the next one out is on a competing carrier.

Second, you could have requested an "upgrade" to an otherwise appropriate vehicle. A Tahoe, Suburban, Yukon, Yukon XL, Expedition...etc., or any of the crossovers (Traverse, Acadia, Enclave) would have worked for you. I would be very surprised if they did not have a single one of those kicking around somewhere.

Third, you could have changed out the car outside of the NY area. You probably passed 20+ National locations between New Jersey and Florida. Understandably that's a hassle and annoyance, but you did not have to drive the Ram the entire time.

And as a side point, the master agreement prevents you from recovering any damages like these, so, at best, anything they would comp you would be goodwill.

Sounds like an unfortunate situation but you won't get anywhere asking for a gas-difference reimbursement. Especially since you can't actually prove anything with respect to an actual damage; i.e., you could have driven the minivan in such a way as to get similar gas mileage on the highway as you did with the Dodge Ram. If you had a Hemi, driving normally you'd be getting 17-18/ish on the highway, and the Ram V6 is actually in the low 20's (it's rated for 25, but I've never gotten one that high) on the highway. Both of those are within the range of a standard Town and Country's low-20's figure for gas mileage.