We have a 2004 Marquis Sapphire.
The water pump, water filter, Red Hat valve, and tee/shut off valve for the ice maker water supply are all in the shadow box located in the rear bulkhead between the water bay manifold and the basement storage forward of the water bay. To access the shadow box, open the basement door foreside of the water bay door on the street side of the coach, look inside on the vertical bulkhead to the immediate right. The shadow box is about 15" high by 24" wide by 7" deep and is only about one foot from the outside wall of the coach. The shadow box is covered by an easily removed door. Get a kneeling pad and it's an easy reach from the ground outside the coach into the shadow box.
Now is your chance to get a water pump that will perform more to your liking, for example, more/less pressure, more/less flow volume, etc.