I'm totally with you on not wanting to do more work than necessary. However, I am not sure how the fill valve could be causing my issue. The valve definitely does SOMETHING... we established that putting the valve into the "closed" position- that is, the position indicated by the manual and by the instruction placard in the holding tank bay- redirects fresh water through the lines back to where the tank is located. What has me confused is that there does not seem to be way for the water to get into the tank except through the pump which would require the water to flow through the pump in the direction opposite of the normal pump flow.
