Something is blocked since the hot and cold pressure come from the same source. Try back washing
your system with a garden hose, but not at the pressure you typically get from a garden hose. You obviously will not want to risk back washing
anything into your potable water tank so disconnect the hose on the discharge side of that tank. You will need to disconnect your water pump at both sides and backwash to there and from there.
Have a bucket collect the backwash so you can see what kind of junk it is that is clogging your potable water system.