There is a Love’s next that we passed on our way through Flagstaff back home to the valley. There is also a Camping World/Gander up there with a dump station.

Hi! I believe the closest dump station would be in Williams at one of their campgrounds. I also know that a few miles to the east on I40 there is a KOA with a dump station for a small fee

There are dump stations in the town of Williams 3 miles away. You might want to check with the RV resorts there, or with the gas stations.