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.
Even if you are on the right track, you'll get run over if you just sit there.
- Will Rogers, American actor