If you have winterized, then you will not need the filter again this year and can discard it. It's typical to get a new one each year. If you will de-winterizing and use again soon, you can throw it in a clean bag and keep inside. Don't keep for too long however as mold and bacteria can quickly develop. If determined to keep and reuse it, put it in the freezer. Replacements can be had from Camping World or via Amazon. Make note of brand and model before discarding or if not marked take a picture and measure the dimensions carefully.
Land of 10,000 Lakes
2016 Forester MBS 2401R
Nights camped 2016: 32; RV to date: 35
2016 FR Forester 2401R
Towing 2014 Honda CR-V