" How do you handle that? Do you leave the gray tank valve open connected to the sewer, and just run the water at the sink until the tank is empty?"
Leaving the gray valve open and running the pump works. Somewhere during our ownership I collected a few PVC parts and made and "adapter" that I could push onto the fresh drain outlet and attach a garden hose. To keep it on, I cut some slits in the adapter so a hose clamp could "squeeze" it to the outlet. With the slits, it leaked a little bit.
My thought was to have a convenient way to sanitize the tank once or twice a year.
With my homemade adapter I could pull in to campsite, drain the tank, fill it with a bleach treatment, drain the bleach, fill it with a rinse, drain it and fill it again. Considering the volume of water, I tried to do it in parks that I though had city sewer connections and not septic systems.