With the pump in place and wiring connected, use a jumper wire from a good ground to the black wire on the pump. If the pump turns on, you're most likely having an issue with the water pump controller. Newmar used a poor quality model. It looks like the photo below. Follow the ground wire from your pump, up behind the wet bay wall and you should spot the controller on the wall. The one listed below, from Amazon, is the same thing, but better quality. I carry a spare.
https://www.amazon.com/Intellitec-00...s%2C208&sr=8-1