Since water and fuel don't mix, why not take several 5 gallon cans and drain the tank into them. Then drop the tank, clean it, and re-install it. Then take a flashlight and look into the cans of fuel... you should see beads or bubbles on the bottom... that's water. Simply decant the fuel off the top of the cans back into the truck, then dispose of the rest that contains water... sure will make for a lot less disposal.
And if you are really unsure, use a clear glass or plastic container to transfer fuel from the can to the tank... the water will be readily apparent. The entire bottom of my fuel cell is covered with beads of water... just a matter of time before they get sucked into the pickup.
steved