Gettysburg is a very cool destination. BTW - as a lifelong Pennsylvania resident, I will caution you on two things as you drive through our lovely commonwealth - pot holes and hills. The state is nearly broke, so potholes are not getting filled as well and as frequently as they should. We are talking bone jarring, tire eating canyons. Use caution.
The hills will give your truck a run for the money, but it should be fairly effortless for the Duramax/Allison combo. Just know you may end up being stuck behind some doofus slowing down traffic towing a 35' TT with a gas powered Expedition.
Finally, if you need to fuel up, try to do so in NJ. Fuel is usually about 5-10 cents less in the Garden State.