I just finished borrowing a friends motorhome and after emptying both (black and grey) tanks the one guage still said 3/4 full. I shrugged this off as an old motorhome with a faulty guage. This weekend I've just purchased a TT that was meticulously taken care of. But this one too, says the black tank is 3/4 full even though the seller said it's been winterized. The grey tank, at first, indicated 1/4 full...after running the water lines to get rid of antifreeze it now says 1/2 full. But I doubt I even ran a gallon or two tops. At this rate it'll be full in another 15 minutes! (I haven't tried to drain the holding tanks yet, as I assumed they were both empty, as per winterizing. It's still in our driveway as I check everything out) Do holding tank guages normally tend to "go"....is there a way I can fix it? or at least know if its working properly?