Hi bobday55,
Welcome to the Heartland Owners Forum. If you start at the sewer outlet, you may be able to see where the 1-1/2" drain pipe joins the 3" sewer pipe. The gray #2 valve will probably be pretty close to that junction.
On our 2011 Rushmore, I had to cut an opening in the coroplast to get to the gray #2 valve. It's about 1 foot to the rear of the drop frame and about a foot or so inward from the frame on off-door-side.