Hi Sjsweet,
Welcome to the Heartland Owners Forum.
The Bighorn 3370rk floor plan shows a 45 gallon black tank, and 90 gallon gray, which is usually 2-45 gallon gray tanks. On most models, the toilet empties into the black tank, the shower and bathroom sink empty into gray #1, and the kitchen empties into gray #2.
What makes you think your shower drain empties into the black tank?