I was just curious. I've had issues with valves leaking (breaking), a mystery water leak inside the trailer that the dealer can't locate, but it's there because I showed them pictures.
I have heated inclosed tanks that they used the same type of switch that they used for the lights.... No way of knowing which why is "on or off unless you turn the lights on. So I "assumed" the switch for the heated tanks would be in the same position as my light switches when on and off, but nope..., they installed that switch up side down. My two 12v were going dead every day.
So with the broken valves, leaks that can't be found, low point drains covered up, and not being able to see my tanks I figure they left that tube off (or didn't connect it).