[Festus2]

Is the actual FW tank drain a spigot/or petcock type or does it have a twist and turn cap on it? I'm not sure what you mean when you refer to it as a "freshwater tank spun fitting drain".
If that is the part that is broken or leaking, you shouldn't have to remove the entire tank to repair it. If you can somehow carefully remove the drain valve (whatever type it is) you should be able to replace it with a similar one and use a special sealant/adhesive that is designed for use in this application. You would, of course, need to drain all the water out of the tank before proceeding with this repair.

The screws that fasten the coroplast to the underbelly should all be inserted from the outside - not from the inside - even those screws around the edges of the coroplast.