. As a result, prices near any of the attractions are always 10-20 cents higher than they are anywhere else in town.

It's even worse near the off-site rental lots on Narcoosee Road...which is something I personally find outrageous since they're blatantly sticking it to out of town visitors who but, unfortunately, it's not illegal.
I sort of disagree with you. Whenever we've rented a car, we have usually found the gas stations ON Disney property to be rather reasonable. Better than home prices. And though we never strayed TOO far from WDW/I-Drive, we found on property to be better than most stations in the vacinity. Also, the prices we saw on-property were lower than what we pay at home. Perhaps there are better places if you really drive around, but time is money - esp on vacation. As I recall, we also drove around Kissimmee a bit and didn't see anywhere substantially lower than Disney. The very lowest we saw, by a few cents was on 535 near 192, and it was sort of an offname station - "Liberty" maybe.
I do agree though that the places right near the rental return places are the worse. If you really fill up at WDW and go right to the car rental place you should be okay in terms of being 'full' enough. You just gotta remember to do it and to allow enough time to pack up, check out, fill up and get to the airport.
Our funny (sort of) personal story occurred the last time we rented a car. We were a bit pressed for time in getting to the airport. My young niece gets carsick sometimes, but we didn't havet time to break up the trip for her, and she got sick JUST before we were pulling into the car return lot! Luckily, she gave us about a 15second warning, and managed to vomit into a plastic ziploc bag! Such a great travel item to keep handy; SO many uses! We all congratulated her for avoiding a disaster by NOT throwing up all over the car, and had a good laugh at what it might have been like had she not been so skilled.
But we did get a little 'stuck' on our rental. The salesperson told us were were getting an 'upgrade,' but the car we got was a PT Cruiser. They may classify it as an upgrade, but I'm reasonably certain it has less trunk space than your typical mid-size vehicle, which is all we really cared about. It certainly had less trunk space then the mid-size vehicle my mom got from the same company. We were a bit crammed with all our luggage.