Welcome to the forum....as you have probably noticed, this is a great place for answers, opinions and friendly people. As far as your black tank gauge goes it all depends on how much you use the toilet. Remember, the shower and sink go into the gray tank. I would think the tank should be reading at least something after 4 days but I would test it again to be sure. It's possible that a wire connection is loose or missing. You can fill it with the black tank flush which takes about 10 minutes. If you still don't get a reading, have the dealer check it out.