I suggest the first attachment you build onto your trailer is a mud room.

I was thinking of making some posts on how to build things next to at trailer. Because, you really don't attach it . The trailer is weak at the edges. There is no foundation there like in a normal house. Further, the trailer flexes and warps due to earth movement and winds, so, you kinda build a box next to it, and then put a gasket/gusset inbetween