Couldn't wait to escape to somewhere with actual seasons and where the weather report didn't include some variant of 'heat' or 'hot' 3/4 of the year. I spent 20+ years in Texas and other southern locales. I didn't want to put antifreeze into the fresh water tank. My plan was to use an empty fitting on top of the tank to drop a vinyl tube into a low spot and try and suck out the rest of the water with a shop vac. The fresh water tank also won't totally drain - there will be about 1/2 gallon left once it's drained. Once we get the antifreeze in the black tank, I plan to open/close the black tanks valve to get that antifreeze mixture through the valve cracks and crevices to be sure that there is no residual water in them to freeze. My plan right now is to winterize the fresh water system and p-traps, and then to pour a about 5 gallons of pink antifreeze into the black tank to keep the residual waste (the stuff that won't drain) from freezing. Our tanks are rectangular and were custom made from 3/8 poly sheeting.
I'd guess that there is about 1-2 gallons still in the bottom of the tank. Any tips on winterizing the black tank on our coach? This will be the first winter with this particular setup, and as much as I've tried it doesn't seem possible to totally drain the tank.