OK, now we know what the problem is. Winnebago bought a shipment of fresh water tanks, had one that held water, and installed it. Do you really think RV manufacturers examine every square inch of every product they buy from third party manufacturers before installing them? Really?
Your beef here should be with the manufacturer of the tank, not with Winnebago who bought it and installed it. I see no Winnebago responsibility for this. At all. There is NO WAY Winnebago could have caught that thin plastic.