I don't have a specific pump recommendation because I'm not sure spending the money will solve the noise concerns. As a first step I would isolate the pump vibrations as they are the major cause of the noise. The water system plumbing transfers the vibration and the noise throughout the rig. A small isolation pad (computer mouse pad) under the pump, and flexible loops on the inlet and outlet to the pump will help to isolate the pump vibration quite a bit. Be sure that the outlet distribution hoses are not in contact with the wall or floor as this will amplify the noise. Some have gone so far as to line the storage area around the pump with foam sheeting. Good luck.I have made quite a few revisions/improvements to my 07 21RS, including rebuilding the storage under the couch and moving the pump to do it. I would like to know where to find a "better and quieter pump".
One thing I did was build new drawers for the RV. I used the same drawer front but made the drawers 22 or 24 inches deep. The top drawer in the kitchen was a tip out tray. I built a 22" drawer, installed runners and now have a top silverware/knife drawer. Check the space over the hot water heater and converter. The only place I have not made the change. I plan to put in 24" drawers there, but first make sure the runners won't interfere with access to machinery. If so, I could always pull runners by removing a few screws.
Again, Question is: does anybody have a recommendation for a better, quieter pump for the 21RS?