$300 one way is sooo far off unless you are driving a Hummer!
I just drove from Carver, MA, driving a full size 4x4 pickup, towing two motorcycles on an ancient trailer and got 20-22mpg. I spent around $300 round trip including my gas in and around FL.
I left full, 1st stop was the Flying J at exit 1 in south Jersey at 2.14, second stop was Roanoke Rapids at Raceway for 2.25 (note here- fill at one of the last exits in VA the price was around 2.16) 3rd stop was in GA for 2.11 and I arrived in Deltona with 1/2 a tank left.
The best price that I saw was in GA around Brunswick for 2.02 but I would not had made it but someone getting 25mpg or more would have.
There is gas in Waldboro SC for 2.11 (stopped there going home)
For those who care, BJ's in Sanford was at 2.14 for members (SR46 - exit 101C) and gas on property at the Hess was 2.25