For what it's worth, here is the recipe we've been using for flushing the holding tank for winter lay-up. We think it is very effective as the last flush is quite clear.
Holding Tank Recipe Ingredients:
2 c. Calgon Water Softener (liquid)
1 c. liquid laundry detergent
Add two cups water softener in 1 gal. hot water. Pour/pump mixture into tank. Add one cup detergent to 1 gal. hot water. Pour/pump mixture into tank. Let sit for several hours then top off tank with fresh (hot if possible,) water and let sit for a few more hours. Even better is to sail with the tank full to thoroughly agitate the mixture in the tank. Pump out and rinse/pump with fresh water.
Tip: Many RV stores carry a wand that attaches to a hose to provide a high pressure spray. Use this in the inspection port of the tank (if you have one) then rinse/pump again with fresh water. You may also be able to go in by removing the vent fitting on the top of the tank.
- Warren & Pattie Updike, Warr De Mar, #62