You mean where do you store the 90?
I toss it in the little compartment where the valves were on our old TCs.
Your camper has separate gray drain? That doesn’t make sense. What washes out the black residoo from the stinky slinky?
At camp, presuming you plan on using water like it’s free (because that’s the point of fhu). Leave gray open so you can use sinks and shower at will. (Or just dump it as needed). Leave black closed, you don’t want black tank just free draining.
When you need to dump black make sure there’s some gray water to flush out after black.
(Unless you actually have 2 drains, which I’ve never seen)
2016 Ram 2500, MotorOps.ca EFIlive tuned, 5” turbo back, 6" lift on 37s
2017 Heartland Torque T29 - Sold.
Couple of Arctic Fox TCs - Sold