I dump and rinse the black and gray tanks, then leave the valves open.
When I winterize the rest of the system, water being pushed out of the lines will drain into the tanks, and then dump on the ground.
The last step is to pour antifreeze into the p-traps, drains, and toilet. Residual antifreeze will drain into the black and gray, and find its way into any nooks or crannies and displace the water.
IMPORTANT - Remember to close those valve next spring...
2001 Coachmen Mirada (Ford F53 6.8L V10) - Toad 2001 Civic
It won't do MACH 2, but I can get a sandwich and take a pee.