You may want to ask on the arctic fox/Northwood owner's forum which is
NROA ForumI have a 2016 and like you the line goes into the underbelly. If you get some good tape, Eternabond or some of the Gorilla underbelly tape, cutting a flap into the coreflex is no big deal, you just tape it up afterwards. Leave one side of the flap attached as a hinge.
If you have to get to the valve you will be doing that anyway. There is no way around it unless you remove the whole coreflex sheet.
Arctic Fox 25Y Travel Trailer
2018 RAM 2500 6.7L 4WD shortbed
Straightline dual cam hitch
400W Solar with Victron controller
Superbumper