Tucquala hit it. On my P/U I Tee'd into the bottom drain line to get as much water out of the tank as I could.
You'll have to get into the underbelly to access the pickup line. If you unscrew the pickup fitting maybe you can add a hose nipple, hose, and brass or stainless clunk weight to the inside, in effect making a bulkhead fitting. You can repair the cut in the Coroplast with Gorilla tape.
I've heard RV manufacturers keep the pickup off the bottom to prevent the pump from picking up any debris that may get into the tank.
