What do you want to decorate? There's not much room in a 139 – I've had two of them. The only "decorating" we really did was to remove the window treatments, which were kind of bulky. In a trailer that small, every cubic inch is precious.
By the way, congratulations on your choice of a trailer. These little guys can go anywhere, and they can fit into really tight campgrounds. What kind of a tow vehicle are you using?