Thanks to all that helped me with this. I installed the shut-off valves on each tank today. Each tank holds 300 gallons and I had about 80 gallons in each tank.
I first taped off the vent and closed all the valves. I have this cheapo shopvac that I use mainly to dewater hard to get at places on the boat such as when I change the engine pencil zincs. A helper put the vacuum nozzle down the fuel fill fitting and wrapped a rag around it to seal it off. We turn it on for about 3- 4 minutes and I removed the plug on the fitting. What I observed is that as the plug was loosen I could hear air rushing into the tank from around the fitting. I removed the fitting and not a drop of diesel come out, not a drop.
Installing the new shut-off valve was a simple matter after I have spent days stressing over this. Thanks again.
The five stages of life: Sailboats, Motorboats, Motor Homes, Nursing Home, Funeral Home.