Rt. 15 - just be sure you don't miss the right-hand turn on to Walnut St. in Wormleysburg. Rt. 15 & Front St. are one in the same until you get to this point. Things are not always marked real well in PA.
From the NY/PA line down to Williamsport Rt. 15 runs together with the "Future I-99 Corridor" so it's all 4-lane limited access. At just about any exit, however, you can get off and find fuel at the small towns along old Rt. 15. In Williamsport you leave the limited access and take the Rt. 15 bridge across the West Branch of the Susquehanna River in to South Williamsport. Lots of stations along here but you also have lots of traffic lights and cross traffic all the way south until you're below I-80 at Watsontown. You're back to 4-lane once you get below the Rt. 15 interchange with Rt. 581 except for the stretch at Dillsburg. Enjoy & safe travels.
On edit: Believe it or not most of Rt. 15 is actually in good shape. Otherwise, yeah, our roads in PA are among the worst in the U. S.