I think the turn taking in the large carrier when you are not in the RV is a reasonable solution to an impossible situation. Of course you don't want your air flow restricted, that would be intolerable. I have used feliway in open window seasons. Maybe it's not AS effective but I did still see results.
I wonder...have you ever tried the "vanilla trick"? I've never had need of it, but I know a lot of people who advise it and swear by it. (for instance when one cat smells like vet and other cats are having fits over it)
This is putting a small dot of vanilla on the nose, chin and root of tail of every cat. The theory being that, since scent is everything to a cat (which we know it is), the vanilla makes them all smell the same to one another, and so are more tolerable of one another.
Cooking vanilla, not perfume, and of course make sure none gets in their eyes.
And Rescue Remedy may be worth a try, too. I think I mentioned it before but don't remember if you said you'd already tried it. The RR can at least help reduce Zephyr's stress. She's precious, I saw your pics in the other thread. Phaedra is beautiful too.