Exit 108 on I-40 in NM is about 300 miles from Williams and has the Dancing Eagle RV park and Casino. There is also a grocery store with a good deli and fuel stations. The RV park is just a gravel lot with back in and pull through spaces but the prices are reasonable. If you want to take the time you can drive 12 mi south on NM23 to Sky City Museum and Acoma Pueblo, one of the continuously occupied mesa top pueblos.
Another 300 or so miles will put you around Amarillo, TX and exit 60 or 63B will take you to 3 or 4 RV parks on the west side of town. Oasis at exit 60 past the Loves south of I-40 on Arnot road is one of the higher rated.