Not sure if it will help you at this juncture, but I had a 2″ union “plastic welded” into the bottom of my waste tank. Same type of tank as yours: HDPE. I then cut the hole in the bottom of my trailer, attached a 90 degree elbow to shoot it out to the side, then attached a Valterra Bladex valve to it. It works like a champ, and with the 2″ whole being directly in the bottom of the tank – it drains very quickly.
The union is a screw type – so if needed, I can unscrew the pipes and stick a hose up there and rinse the whole thing out.