If it backs up in the shower it's TOO full. Unless we're on the road, we leave the gray water valve open while hooked up in a park. Never had a backup that way and never had black tank emptying problems either.
2009 Chevrolet Silverado LTZ 1500 4x4 5.3L
2011 Kodiak 281RLGS travel trailer
2011 Egg Camper
2010 Chrysler Town & Country
Good Sam life members
Bob & Grace professional retirees
I ignore the idiot lights. I have filled it all the way up into the shower several times. Unless your plumbing is bad, there is no problem. I purposely did the same with the black tank, filled it till the toilet couldn't flush, again, no problem.
Taking naps regularly prevents old age, especially napping while driving!