Not really familiar with your 1640esp but pop-ups usually don't have winterizing kits. If it has one it will be located on the intake water line between the water tank and the pump. Both the tank and water pump are usually located under a bench seat. Have you checked the screen filter on the intake side of the water pump? It may be loose, a missing or damaged o-ring, or have a crack in it causing the pump to suck air. Also check the fittings for loose clamps.
Richard & Jill
2014 Flagstaff 832IKBS Classic Super Lite
2018 Silverado 1500 Crew Cab Z71 4WD All Star Edition
Camping since 1989, seasonal since 2000
Car Shredder Op/Tech, Scrap Metal Recycling - retired
I deserve hazard pay when camping with my relatives.