Twist-on sewer valves…

No concern about possible black water mixed into the gray tank/lines?

Two of the mods Dad and I did to his 5th wheel when he started full timing. Install a third valve near stinky slinky hookup so at least 2 valves had to be open for anything comes out.
The 2nd was a second drain, with 2 valves, about 3/4 up on grey tank. With both open, grey water would bypass the main valve, and if the added valve was open, thru stinky. Close the lower, the water exits thru hose bib. Close top valve, and could back into shower.
On FHU site, he would run small hose from hose bib to sewer, or where allowed, into bushes. Live as normal, (normal, for him, was dump dish pan down toilet. Food bits and soapy water help move the cornbacked rattlers) until black full. Hook up slinky, drain black, have plenty of grey to flush, stow slinky.
If he had guests on a E/W site, shower fill tank, he would drain a few buckets dump into black tank.

msmith1.wa wrote:
I leave mine on all the time. Prevents surprises from the other two valves if they were to leak.

Exactly the twist on valves work better than a sewer end cap

I leave mine on all the time. Prevents surprises from the other two valves if they were to leak.

Yup, I've carried one for years, in case of emergency. Still unused, but ready when needed.