I notice that when I turn on the water in the tub, the water drains as it's running. However, once I turn the water off, most of the water drains, but some water just sits in the pipes indefinitely. The drain doesn't seem clogged because the tub never fills up with water, but there is always just a little water sitting in the drainpipe that I can see from the top. Is this an RV thing? Is this normal?