I have a 2015 28TQDL. Is the fresh water tank drain glued into the tank or does it unscrew? It has treads on it, but it also has purple stuff on it making me think pvc glue. After I blew out the water system it got down to 25 degrees that night, the next day I had drips coming down the drain tube. It an insulated bottom so it hard to see very well.