Here is my experiences with gas along I95. Now my last trip was April so things may have changed, but for the last couple of trips, these areas held true for lowest pricing.
Southern NJ, Exit 1 Go to the Flying J. It's always .10 cheaper than the turnpike and the other truck stops in the area. Maryland is .10 to.20 more expensive.
Southern VA Emporia. It's been .05 cheaper than Roanoke Rapids, NC the last couple of trips. However, in my trips in 2005 to early 2006, Roanoke Rapids was cheaper.
Walterboro, SC. The El Cheapo which is about 1/2 a mile from the highway runs .05 cheaper than the stations right next to I95.
I always top off in GA, but I have been finding either exit 7 or exit 1 has been cheaper than the gas stations at exit 3.
In FL, if you can make Disney property, gas at the Hess stations there, if not, stop in Seminole County. The gas usually runs .05 cheaper due to not having the extra .05 a gal tax.
The above stops are between 300 to 350 miles apart. That's my range on the highway.
YMMV