Rt 15 is a good route. Almost all divided highway, Some long hills, but the same hills that you'd get on I-81, just a bit further west! I've traveled it in both directions, both towing and not towing. As I recall, the stretch between Williamsport and I-86 can be a bit sparse finding fuel, especially at night, so I would suggest filling-up in the Williamsport area.
2014 RAM 2500 Laramie 4x4 Quad Cab w/Cummins Turbo Diesel, 2009 Rockwood 2607, and 3 'yaks!
300W of Suntech Solar, a Rogue MPT-3024 Controller, and a Xantrex PW2000 Inverter