My fresh water tank didn't fall off because with an open underbelly I noticed how much it sagged the first time I filled it at home. Crawling under the trailer I then noticed the pump intake was connected halfway up the tank, which explained why I could only draw 1/3rd the capacity of the tank before having to refill it. The first couple of trips we took until I could get the sagging tank issue addressed we did with just a minimal amount of water in the tank ... had it been full during travel I have little doubt I would have left it lying on the highway. :M My dealer did an effective job of supporting the tank but I've since added a 4'x 2' piece of plywood to further enhance support and can now travel with a full tank when need be.
In conversing with my dealer's service manager he did acknowledge they've had several completely drop off. :E For my own part I did ask that pics of my own tank be sent to Coachmen in hopes they might address this issue but I notice new 2017 models are exactly the same.
The black and grey tanks are securely mounted and even when full have shown no signs of failure in 3 yrs of towing.