i was just messing around with the water pump on my 1977 overlander. this is how my trailer is set up
outside underneath the trailer there should be a steel plate with a bolt in the center of it. when you remove the bolt you can take the plate out and gain access to the oulet on the water tank and the water sensors. mine has a hose that runs from the tank to the water pump then from the water pump to the larger of the two copper lines that run toward the rear of the trailer there should be two large copper lines on is bigger on my trailer and that is the cold water supply for the kitchen sink and bathroom sink and so on. it ties into the water heater. the copper line with the the copper T's going to the sink should be the line you will tie into the outlet on your pump. my trailer has a water filter system on it aswell it is on the cold water side. aslo you should try tracing the lines back you can follow the lines on my trailer by looking underneath the sink, outside compartments, under the right side bed, rear compartment, and behind the toilet were the laundry bag is and inside the cabnet in the bathroom were the water heter is located. the water heater is labled in and out in should be cold water from the tank. by doing this you can figure out what water line does what.
yours might be set up like mine i would trace the waterlines to figure out whats what.