It appears that you have what they call a "pressure regulator" in the input, just below the 90 degree elbow (just below where your arrow is pointing).
These "regulators" like this

really don't work very well and mostly just limit flow rather than pressure.
I strongly suggest a real pressure regulator like this

available through Amazon and just about any RV outlet, that does actually regulate the pressure and protect your RV's plumbing from over-pressure that you may encounter at some camping sites.
I have never heard of an extendable tank vent pipe. I'm guessing you may have pulled the pipe out from a connection, possibly clamped, somewhere between the roof and the tank