[beeje]

Maybe I can be of some help here since I have had the entire coroplast cover off of my unit. The following info applies to my 2011 unit, so not sure about the newer ones.
My fresh water tank is simply a one piece molded/ flat bottomed tank.
There is a small lowered/recessed area where the tank drain is located.
The water pick up connection is off of the lowered section.
Because of the weight of the water, the tank sags quite a bit on the bottom which prevents all of the water from getting to the lowered section.
After each trip I open the drain valve to drain the tank while driving so all the water flows to the drain valve.
If you drain it while sitting still you will have several gallons that will not drain out.
As for the size of the tank, I think I calculated that with length/width/height measurements, but I can not for life of remember what is was.