Another issue that comes to mind is weight. Between the piping, the foam, the fluid, etc, etc that you would need to install your system on the ceiling, you're talking about a lot of weight. All that weight may overload what the trailer manufacturer designed that roof to hold. In addition, even if the roof can hold that weight, you'd be making your trailer top-heavy which could effect the handling.
Why don't you install your system in/on the floor? Seems like that would overcome many of the concerns listed here.