Nationwide is not on our side

This really depends on what your policy says. Read it closely and see what exclusions are included. I have seen in my law enforcement days, where insurance companies paid for cars, damage, and repairs where a car was driven through a garage wall and into a swimming pool. But, with extended warranties, you have to read the exclusions. The vent pipe not going through the roof is a manufacturing defect. No a maintenance issue in IMO.

Leaving the water on in the black tank flush is an accident. Insurance is supposed to cover accidents. If everyone was perfect and we lived in a perfect world, we would not need insurance.