first time winterizing my 5th wheel

If you have a Black Tank Flush connection, get some anti-freeze in there too. There's a Anti-Siphon Vacuum Breaker/check valve, usually plastic and situated under the bathroom vanity that will likely still have some water in it as well as the Pex pipe that it's attached to. They sell hand pumps to winterize this part but I just take a piece of garden hose and screw it on to the flush connector and then use a funnel to pour anti-freeze through the hose and into that area. Lift the hose above the connector until it flows in and that should be good. Also, don't forget about the washer/Dryer plumbing if your rig is equipped for them.