To me it sounds like you have a flow restriction somewhere. Is it the same for both hot and cold water? If so probably something in the line between pump and where it 'T's off to line to hot water heater. Also possible problem with suction line from tank, some thing in line or either obstruction at exit from tank. You might be able to take suction line off tank and look into fitting. Also try a hose from a gallon jug of water to pump and see what happens, that will tell you if it is before or after pump.
RESPONSE to post below.
On my trailer it is just a "hose line" that goes between the pump and the water tank. It has screw type clamps on the line. As long as it is accessible it should not be to hard.