If you are boondocking on BLM land, there are usually dump stations at the check-in area where you can either drive your rig to dump or take your "blue boy" to dump. Depending on your permit, you may have to pay to dump. The LTVA permits in Arizona include use of the dump stations and allow you to fill your water tanks. You can also find dump stations at some service stations or county or city sites. Usually there is a fee and sometimes they are several miles from where you are camping.
Some people don't like to move their rigs once they are set up for the season and will contact a service to pump out their tanks. Availability of these services will depend on how far away from civilization you are. These tank trucks are called "honey wagons" and will empty your tanks in a few minutes for about $20 or $25. There are also services that come to fill your water tank for a fee. Because of the fees and inconvenience, water conservation is extremely important.
When boondocking in other places, it is wise to scout out possible dump stations and places to fill water tanks before heading out. Always head into a boondock area with empty grey and black water tanks and full water tanks.