If you have a 76 Argosy 24, you should have two tanks. We keep both valves closed When we are camping, then every couple of days (there are just the two of us) my husband drains them, first the black and then the gray to flush out the hose, then closes them again until the next time. We don't wait until anything backs up before we drain the tanks, we just do it as a matter of course so we DON'T develop a problem. (This is from experience, when we were newbies we left it too long and it backed up in the shower). As I remember from our manual, it does not recommend leaving the valves open all the time.