That third line is simply a double jacket braided hose....it's crap! With the cinch clamps that are used in pex tubing you can't get a proper tight grip on it...I had many issues with mine in our new 2014 trailer. Had to repair a leak and when I finally got what seemed to be a good seal...a couple hours later I noticed the line was swelling. Turns out water was getting between the two layers and started to swell the outer layer till it burst.
I talked to a local RV repair guy and he told me that braided line is a headache and he recommended I remove that section that was giving me grief and replace it with regular pex tubing. I followed his recommendation and problem solved. There was a post in here in early summer whereby a poster posted a picture of this braided tubing where the inner layer had collapsed over the course of the winter while it was in storage. Unfortunately there are sections of this line all over in the trailer and much of it is very difficult to access.
2011 GMC Sierra 2500HD
2014 Rockwood 8289WS, Diamond Pkg.