From Robesonia, PA to Madison campground in Yellowstone there are two direct routes:
1. I-39 Wisconsin to I-94 to Billings, MT to I-90 to Livingston, MT to 89 south through Yellowstone to Madison. This is 4-1/2 days @8 hr/day for 2160 miles.
2. I-90 across MN and SD to Cody, Wyoming coming in from the east entrance to Madison campground. This is 4-1/2 days @8hr/day for 2120 miles.
I-90 is a more pleasant drive than I-80.
Of course, there are many other routes depending if you want to stop any particular place along the way.
I would highly recommend you also stay at least a few nights in Grand Teton Nat'l Park, also. It is completely different from Yellowstone...perhaps doing it on your return.