As far as I can tell the only differences seem to be in fit and finish. Maybe they have differences in what is standard and what is optional but there is no literature on the Parkway that I can find to determine that. They are both 22 ft Sprinter vans with the same floor plan options. What little their is on the Parkway literature seems to be the same as the Interstate. All the Interstate pictures and literature I have seen all have a substantial rooftop tube rack. The Parkway pictures don't show that.
I looked at an Interstate at a dealer. It was the plan with the mid dinette and kitchen and bath in the back. I'm not sure I like that layout. I did like the cherry laminate finish cabinets, gray leather upholstery and use of aluminum finish on the interior.
2015 Sprinter Class B Camper Van
(Former 1971 vintage Airstream Owner)