We spent most of the day today reinstalling a dump tank that had fallen out of a travel trailer. In the event, that sound like a rare event, it is not. Many manufacturers use a flanged tank and the flange tucks up under a metal rail running side-to-side. It looks stout and it is provided you don't run down the road with full tanks. If you do that, the shear weight of the water bouncing up and down deflects the flanges and the entire tank drops right out ripping off the dump lines as it goes.
Really a heck of a job getting it all back together and on this one I had to cut open a wall and then install an access panel to cover the opening. If you have to run with full tanks make sure they are support underneath with straps or better still a piece of plywood strapped up against the tank, would be my advice.