"...leave mine open all the time..." Do you have the DTE gauge and not get an error/check engine warning?
I just had an RDS 60 gallon tank, gravity feed, installed in my 2021 Ram. I haven't tried leaving it open while driving, but I just tried to fill the OEM tank from the aux tank, and the OEM tank builds up pressure and stops filling until I push the OEM flap. Then air poofs out and it starts filling again. So I'm guessing if I fill while driving it will work better(?). But I've heard that with the DTE gauge I'll get an error message. I'm also thinking so what if I get an error message... just clear it out with five starts, right?