Winterizing Fresh Water Drain

I've searched on this site and googled but I can't find any information specific to this issue. I have my 5th wheel all winterized including the low point drains and valves. I drained out all of the fresh water and closed the drain. The next day, I opened that drain and maybe a couple ounces of water came out. The same thing the following day. I'm assuming there is drops of water in the line finding its way to the valve but how many times should this happen? Is there a way to winterize that valve without getting anti-freeze in the fresh water tank or do I leave the valve open all winter? If I do that, how do I prevent spiders etc. from finding their way into the tank? Thanks!