Back the RV up to the septic-tank or sewer-system access. Open the access lid.
Hook the flexible hose up from the bottom of the RV to the access.
Pull the valve for the sewage tank and dump its contents. All RVs have two tanks: one for waste from the toilet and the other for dirty water from the sink and shower. Typically the sewage tank is black and the dirty-water is black but consult your RV's owner's manual to be sure. Close the valve when complete.
Pull the valve for the dirty-water tank and empty it. Close the valve when complete.
Unhook the flexible hose and return it to its place underneath the RV. Close the access lid.