There's no convenient way to get from Chambersburg to Breezewood when towing. You either go over the mountains or around.
Another option for you when returning from Gettysburg, take US 30 west to PA 997 north, just before Fayetteville, PA. Follow PA 997 until it puts you on the PA Turnpike at Exit 201, Blue Mountain. Then take the Turnpike west.
PA 997 is two lane but, good road and no big hills. Trucks use it to go from the Turnpike to the Letterkenny Army Depot.
You will pay a little more in tolls on the Turnpike.
That's the route I would take if traveling in our Southwind.
Enjoy your trip to G/burg.