The tub is indeed the low point in the system, and when the gray tank fills up, it backs up into the tub. This is not a mistake - this is what is supposed to happen. As JoeAllen and B&D pointed out, this is your ultimate signal to dump the gray tank!
Apparently the connection between the tub and the gray tank is a set of solvent-welded fittings (not screwed together), and the solvent welding was not well-performed by the factory on a few units (including mine). If this is your situation, the connection will leak when the gray tank fills up. Fixing this is a more-than-major pain - see the thread by Al R Hall who is struggling with it at this time.
My suggestion - if your TM is still under warranty, and you really really REALLY want this fixed, take your TM to the factory. They will do it. But otherwise, just follow B&D's suggestion, and dump, at least partially, before it gets full.
Bill
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