Hi DonM,
Welcome to the Heartland Owners Forum.
Someone with the same floor plan is likely to chime in. But another way to go is to remove the coroplast mounting screws at the corner of the drop frame near the sewer outlet. If you take out 4 on each side, you'll likely be able to see the valve.
Better to use an impact wrench than a ratcheting socket wrench. It's easier and less likely to shear the heads off.