Add some antifreeze to any thing like the water tank, plumbing you can not completely drain, the shower sump pump box, and any other water holding areas that you can not completely drain or suck dry with a shop vac. Personally, I would not add antifreeze to the bilge if there is no water present unless you are expecting to store it outside where it is likely to accumulate water and freeze.
When considering your water system, some people prefer to blow the plumbing out completely with compressed air, if available, to eliminate the antifreeze odor or taste the next season. The Four Winns owner's manuals refer to a process of treating the system with a mild bleach solution, flush, then a vinegar/water solution, flush, then fill and use. We have had good luck with this, but remember that seasonal use of a drinking water system can lead to issues where you might not want to drink the water unless treated or boiled. This might lead to consuming "other beverages"
