This is a major problem waiting to happen on these trailers. Like you, we have a residential fridge in our Cardinal with icemaker, and through the door water. Love it. But....I can see that a lot of people will have problems with that thin waterline freezing. That line goes out of the heated basement, into the outside to re-enter the slide with the rest of the electric, gas, and tv coax. If you do not winterize it correctly, or dewinterize before freezing season if over, with the assumption that you'll just run a heater to keep the trailer above freezing, that approx. 6' of thin line is subject to the elements and splitting if it freezes.
I'll be dewinterizing our's next week in preparation for spring break trip, but I'll not flush the antifreeze out of this line until May/June time frame. I may need to come up with some sort of low-pressure pump that I can plumb onto that line to be able to winterize just that line as needed.
The shutoff valve in our cardinal is in the water bay, and is a simple push/pull valve.
Dave
2016 F350 Lariat 4x4 FX4 SRW CC SB 6.7 Magnetic Metallic
2017 Forest River Cardinal 3850RL