Re: Tank #1 vs Tank #2 in a Bigcountry 3450 (2010)
You could add water to both tanks, if they're not already in use, and add some food coloring to one (via either the bathroom or kitchen sink). Put a clear adapter on the sewer connection. Pull one handle, then close and repeat for the other handle, and when you see the color, you'll know for sure.
I don't know about BC's, but on my BH, Gray 1 is the bathroom, Gray 2 is the galley.