There are several different ways that trailer roofs can be built - you'd best know how yours if built before starting. (Usually this is in the brochure.)
Some are framed - for those it's usually pretty easy to find the trusses by just pressing with your foot and feeling for the spots that don't give, then feel to either side to estimate where the center is.
Some are laminate - you can mount it pretty much anywhere. Under the rubber will be a layer or two of very thin plywood glued to foam. These roofs are generally quite strong and will hold anywhere.
I'm sure there are other ways to make a roof, but those are the two I'm familiar with.
