Two things, coach.
First gas is cheapest between Ocala and the Turnpike turnoff along I75. There are also Ocala exits with cheap gas. Gas on the turnpike is expensive. Ocala to Gainesville is also high. Lake City is a downright rip-off. To Cousin Ed's point, though, the gas on Property at the Pilot stations (formerly Hess) are about 5 cents higher a gallon which I don't think is too bad for a Disney price hike. They have 3 stations on property - across from DHS, adjacent to Disney Springs, and the aforementioned Auto Center near the MK toll booth.
The website (and app) gasbuddy.com is a good resource:
http://www.gasbuddy.com/
Type in Ocala and use the map view to scan around. Lowest in that area today I saw was about 1.96 and the Disney stations were 2.02. Every exit has a big gas station like a Pilot, Racetrac, etc. (no Flying J tho) with high clearance and lots of pumps. How busy they are depends on time of year and time of day. The gas cost calculator is a handy tool on that website also.
On the idea to get off Hwy 429 at exit 19 to get to the Fort, don't do it. Plain and simple - no. Even if Google Maps routes you that way.
I used to use that route 15 years ago. It was a lot of empty fields and 3 traffic lights. I went home that way this March 2016 from the Fort and there were tons of stop lights and traffic. I regretted doing it and won't again. You are much better off rolling unimpeded down Hwy 429 to exit 8 and follow the Western Way onto Disney property near Coronado springs. There is one toll booth on Hwy 429 that way. Take a left onto Buena Vista Drive and left again down onto World Drive and go through the MK toll booth to the Fort.
My two cents.
Bama Ed