Go to Keystone montana Owners Group Facebook page.
https://www.facebook.com/groups/2740...1120790278328/
Jim Vititow had this problem and found a pinched water line.
I'm guessing the best way to find this would be to push an electricians fish tape down the suspect pipe and see(measure) where it stops. Then cut open the insulation an have a look.
https://www.amazon.com/Puller-Throug...1958693&sr=8-8
You would need to do this from each joint/branch of the affected pipe.