Hmmmm, I have never seen calcification on plastic water pipes, not to say it could not happen. My other guess is it is residue from assembling the system in the first place, by not cleaning up any debris when the pipe was cut and just plain dirt when the coach was made. Now, if you sanitize the system about once every 6 months (assuming full time use) or at intervals of over 30 days of non use, the chlorine solutions used might eat up that stuff. Another way to prevent this is to pre-filter the incoming water. Your RV Regulator should be the first thing you put on the park water outlet, then the fresh water hose which connects to the RV. At the regulator, you should have a screen washer and the point of entry into the rv should also have a screen washer. Hope this helps.
Monty & Janet - 2007 Alpine APEX 40 MDTS
S/N - 75715 - Retired - Master Certified RV Tech