It would help an awful lot to know what year/modle of trailer you are working on.
If it is a 70's vintage, then the order of assembly is as follows: The skins are put in place from bottom to top, overlapping as you go up. When you get to the center section, a set of rail/channel trim is riveted to the higest edge of the last sheets before you get to the center. The center skin is then slid into the channel, and this hides the rivets that hold the channel in place.
So, start by removing the center/ceiling section, then remove the channels/rails, then you can get at the rivets for the next layer of interior skin.
If you don't have a 70's era trailer, then the above may or may not apply.