From my experiences (YMMV) gas prices are best in South Jersey, Southern VA, SC and GA.
When I was driving from MA, I always gassed up in NJ at the Delaware boarder, Skippers VA or Roanoake Rapids NC, Waltersboro SC and Exit 3 in GA. These locations were usually the better prices and gas stations were right off the highway.
Pulling a trailer will make for more frequent stops but I would avoid Delaware and Maryland (at least the service stations ON 95) if possible and make sure that you have sufficient fuel to get a decent way into VA. I once needed fuel in the Alexandria area and had a very difficult time finding gas in the wee hours of the morning.