A must see in the Amarillo area is Palo Duro Canyon State Park. It's about 30 minutes from town. Cadillac Ranch is right on the I40 east service road. While you are there let the Big Texan Steak house pick you up at your site and deliver you back when you are finished with your meal. They have 14 limos and the rides are free unless you want to tip the drivers. I doubt that they will come to the park but there are several RV parks in Amarillo proper.
We were out there roaming around in May and wish we had spent more days in Albuquerque. It's just beautiful country.
We skipped Santa Fe and felt that Sedonna was little more than a tourist trap other than the natural beauty.
Homolovi State Park AZ is just a few miles from Winslow if you like standing on the corner. It's only 2 hours from the GC and has ruins from the 1200s believed to be the ancestors of the Hopi indian tribe.
Lake Havasu City is just across the river from CA south of I40 and is home to THE London Bridge and a fabulous lake of the Colorado River.
Don't try to do it all, just soak what you do in and enjoy the trip.
Most of old RT 66 is gone or in shambles, cool to see, but somewhat disappointing.