Here's a stock trailer for your 1:64/S scale farm.  The trailer is designed to work with a model of a 5th wheel hitch: https://www.thingiverse.com/thing:3016919#google_vignette

I must add a disclaimer; I did not design the original model.  I found it quite a while ago, and only recently modified it to fit my needs.  For some reason I couldn't upload this as a remix.

I hollowed out the trailer and made the door swing, as well as adding fenders and wheels.  I also had to upscale the model in order to make the nose long enough to swing on my longbed pickup truck. 

This model was extremely straight forward; no paint, and almost no glue except on the door hinge.  My axles and door hinge were all straightened paperclips as per usual with my models.  The wheels were modified versions of the wheels I designed for the gooseneck flat deck trailer: