The tank supports the bottom of the tank, correct? The floor above the tank is what is spongy-I think it's two different issues. The top of the tank does not support the floor. Is it possible that water entered the door area and soaked the chipboard floor?
So, finding the leak and fixing the floor might be the first thing to do, then, tackle the tank bottom support issue.
And no, I don't think that the factory should have used such meager tank support on an almost new trailer. Send some photos to the factory, maybe they can help!
Bill, Kathi and Zorro; '05 Beaver Patriot Thunder
2012 Sunnybrook Harmony 21FBS (SQEZINN)
2007 Jeep Grand Cherokee Overland