For a fall tour in the U.S. I'd go for the Pacific Coast. I don't think mid-September would be too late to start in Vancouver, BC, and head north to Lund, cross over to Vancouver Island, and start south to Mexico. By mid October you would be far enough south that cold weather should be less of a problem. However, it is the coast, and it can get wet anytime of the year.
This is a good site, if you want to check historical climate conditions for an area:
https://weatherspark.com/averages/28...he-Netherlands