I have a plastic shutoff valve on each tank drain. If I recall they are from Camco, but I don’t see them for sale now - theirs is now brass.
I have something like this
https://a.co/d/0R1Y267
Or this
https://www.etrailer.com/Water-Shut-Off-Valve/Valterra/A01-0125VP.html
I drain via a hose, or...