On my 2015 19' FC the water pump is on the 12 volt
DC circuit #5 Blue wire 15A fuse. Also on that circuit is the Closet light and compartment light.
If the pump commutator/brushes on the water pump's motor were getting worn or defective they could cause the motor to not run smoothly and to pulse the current draw on that circuit.
Another possibility is that the vibration of the motor is affecting a loose connection on that circuit.
One way to determine if it is just vibration would be to take the motor's two wires, disconnect them and wire them directly up to a battery (either the ones in the trailer or bring in one and set it on the floor. Before disconnecting any wires, make a drawing of the wires and mark each wire to make sure that you put it back correctly and that you know which wire is the motor plus and minus to connect up to the battery in the right manner.