I would consider it a factory error if my shower was plumbed into the black tank and I would either fix it or hire someone to fix it so it was plumbed correctly.
I filled my gray water tank taking a shower this morning. It caused a backup in my shower pan. I can only imagine the mess if my shower backed up from the black water tank.
We have a FR 30 ft XLF hyper lite toy hauler. We have owned trailers for 30 years and have never had the shower drain into the black. Forest River won't own up to it or fix it. The dealer says it's a factory defect. The problem is when the black tank is full it backs up in the shower. Disgusting! I have never heard of anyone having this issue.