I'm assuming you're talking about Montco,
Montgomery County in Pennsylvania, about an hour north of Philly.
Starting from
Springfield (Illinois), you'd probably be looking at somewhere near
Columbus for a good midway stop. There should be plenty of pet-friendly hotels there like Comfort Suites or La Quinta, or you can find an Airbnb.
Oh, and if you're in Montco, hopefully you have time to visit
Valley Forge National Historical Park or go shopping at
King of Prussia. There are a lot of great fall festivals going on too!
