There most likely is a pressure regulator that came pre-installed in the rig. It has probably failed. Do you get good pressure when you switch to the water pump and storage tanks? If so, the problem lies between where you connect the hose and the valve that switches from city water to fill (it could also be that valve). If you have bad pressure from the pump also, the problem is further in. Could be a crimped hose, debris clogged at a junction or there might be yet another filter in the line you haven't found.