The drive down is not bad but the drive back is bad. The reason is, if you leave by 8am you hit DC around 12noon which is ok but on the drive back (assuming you do each way over two days) you will hit DC around 5pm and it is a bad drive from Alexandra to Baltimore....
Fuel wise it is not too bad. The prices are right around NJ prices or 20 cents less then PA prices. Get the flyingJ/Pilot frequently travelers card. It is free but gives you a cent or two discount plus you get the cash price using a credit card.
Here is what we do... NE Extension to the blue route to I95 south. I take the beltway around Baltimore (I495 i think) because i like driving along the Delaware River. We take the tunnel though Baltimore. The outer beltway around DC and then the beltway around Richmond. Doing it this way we hit nearly no traffic until we hit Orlando. We stop the first night in Florence SC and then stay a second night in Kissimmee. Not really necessary to stay outside the second night other then we like to check into Disney early (8am) on the check in day. You will hit Disney around 5 or 6pm so you could go right to Disney the second night. Once you get below Va the speed limit jumps to 70mph so it is a descent drive. Pay attention to your speed, each state had a bus load of police out giving tickets. In SC alone we counted 33 police...
Fuel stops.... Carmel Church, VA --- Latta, SC --- Kingsland GA (raceway) ---
Walmart on Vineland Rd Kissimmee (cheapest of the lot if you use a gift card from the store - gives you the cash price). I can't tell you the prices as we use Diesel and i honestly never really look at the price...I stopped looking once we leaped over $2.25 /gal.
We do this twice a year so if you have any specific questions, feel free to ask.