As stated above the Fly J fuel station has dump facilities that are free.  Some Rest Stops along the Inter-State Highway system have dump stations.

State campgrounds have dumpsites that are free, along with private campgrounds.  Private campgrounds may charge for dumping if you are not staying at their site.  Some churches where you may park have dumps.

The Amazing Grace properties in Tennessee and Florida have dump facilities.

There are publications, and maps, and mapping software which will sometimes list dump stations.  You will need to plan your “dumps” to ensure you don’t have problems.