Usually a hose fitting with a valve on it to suck that yukky stuff into your water system. Getting close to fall again so all of the posts on winterizing.
I just blow my system out with a 12V air compressor I carry with me. I winterize and de-winterize several times a year. On the road if the temp drops below 10 degrees I winterize. Then just dump rv anti freeze into all the traps. Takes about 20 minutes now because of the washer. If you do blow out lines always leave one faucet open during the process.Plus, make sure if you have any water filters remove the housings and filters afer blowing out the lines. You should remove those filters anyway even if you pump that yukky pink stuff through you lines.