I just dewinterized my boat last weekend. When I left it 6 mo ago the toilet was half full of about 50/50 RV antifreeze. When I looked at it Sunday a fitting had leaked a little antifreeze and it puddled below the toilet. It was semi-liquid as all of the water had evaporated but it easily wiped up with a wet sponge.
So drain your washer, pump a couple of gallons of RV antifreeze through it and you will be fine next spring when you rinse it out.