Why are you waiting for the water to back up into the shower (or sink)? Does your level indicators work? If the indicators are working and the gally or grey tank shows high level drain it/them, you shouldn't need to let anything back up I for one am not following the logic perhaps you can elaberate for us. Let your black tank get at least 2/3 full before draining.
Dale & Terri, Lulu & Tiki (our Chihuahua's), New rescue puppy Prince - Pom/Pug mix.
2013 Tundra, Double Cab, 5.7, TRD Off-Road, 4X4, Full Tow Package
2013 Wildwood T26TBSS - Sold
2000 Gulf Stream Sun Voyager M-8357 MH, Ford Trident V10 Gasser, 35 foot.