Tank capacities

Strange thing, it backed up in the kitchen sink, not the shower.

Ours does as well.  Sink first, eventually the shower.  I think there is a HEPVO trap under the shower instead of a regular P-trap.  It is supposed to prevent sewer gas from flowing up into the RV but the design also works to some degree like a back flow preventer.  So the easiest path for backed up grey water is the kitchen sink which has a regular P-trap.

Thanks to all for your helpful replies, we're all in this together.

  Lynn

The drain comes out the side of the tank not the bottom so 1 or 2 gal. could still be in the tank when drained.

Yes that is par for the course, I think with everyone's fresh water tank.

When I drain our fresh water tank at home, I park the rig on our steep driveway slope to get most of that left-over water, but it seems to be less than one gallon.