There are three Exxon stations on WDW property (still called Exxon Tiger Marts, so far as I know). I also know there is a Shell station (off-property) near the Lake Buena Vista entrance, and many other chains are everywhere, of course.
The Boardwalk Exxon was $1.53 last week. I filled it up before going to the airport, it was still above full when I returned the car. National does not ask for a receipt, I heard Dollar sometimes does (to show you got gas near the airport).
Good gas station near the airport that should satisfy any rental company.
Leaving WDW use I-4 east (northeast) to 528 (Beeline). Get off Beeline at Exit 9 (Tradesport). Turn left and go under the Beeline using the left lane and the gas station is on eht left. Reasonably low prices.
When you are this close to the airport you can take the gas pump nozzle out as soon as it clicks off, don't round up to the dollar or 50 cents.
Don't cross back under the Beeline and get back on it. Instead take McCoy which parallels the Beeline and skips a toll booth. Becomes Frontage Road and at the next light is Semoran Blvd which is the north entry to the airport.
I used the Exxon station outside the MK. The price was lower than some off-site stations. The tank was still full when I returned the car at the airport.