The assembly method is the way these parametric CAD systems work. Create the "sticks" of tubing or channel you will use, then assemble them into the trailer. Alibre, Solidworks, Pro-E, Inventor, etc. are all pretty similar in this regard.
Using something like AutoCAD, you would draw all the pieces in the same sketch. Using these 3D CAD packages, you draw all the components separately, then put them together in another file.