try going to
www.mousesaver.com and looking at the car rental discount codes there...some only apply to FL rentals, but many others are good everywhere.
Here are a couple of comments on rental cars...just because the reservation says Jeep Liberty (or similar) please know that the chances of you actually getting the exact vehicle are slim to none....out of my last 50 car rentals I would guess less than 5 have been the exact vehicle I reserved...so don't count on getting a Jeep Liberty! They may well give you some other small SUV.
Second and this is purely a personal opinion....if I was renting for a drive from Nashville to Orlando and back that would not be the vehicle of choice for me....the luggage would have a great trip, but the rest of you will be pretty uncomfortable for the long haul!! If you have more than 2 people the back seat leg room is less than in a compact car and the front is something less than ultra comfortable. Not to mention the gas mileage is less than optimal. I know that is a personal choice and others may really disagree, but you might want to go for something with more comfort and less flair for such a long trip!!
Finally...you might want to watch airfares... we have flown from Nashville to Orlando the past 3 falls (October or November) for under $100pp round trip including tax and fees. If there are just 2 or 3 of you it may be more cost effective to fly than to drive with the price of gas +rental car.
Oh and yes unlimited mileage is just that...USUALLY, however be very careful to read ALL of the fine print...because several rental car companies (Dollar for sure and others I have heard) will impose a 150 mile per day max if you live in the area of the rental...for instance when I rent from a local Dollar agent my "unlimited mileage" is limited to 150 miles per day , however if the guy behind me in line has just gotten off a plane from CA he can rent the same car at the same rate and his "unlimited mileage" is really unlimited!! (and yes they know where you live because they look at your drivers license). I have not had airport locations enforce this rule but off airport locations do it on a regular basis. I am assuming the airport locations just don't bother because almost everyone coming in has just arrived by plane...but be sure to read the fine print carefully.
Finally you might want to try bidding for the SUV rental on Priceline. I think you could do some bit better than your current price and would avoid the mileage issue, because all of their rental truely are unlimited mileage!