I would attempt this method (I do believe that this totally doable in Blender - could be wrong though - I want to get you thinking with this limited example though, because it may spark some ideas for you):
Your rotational pivot center will be at the center of the axle being dragged, and your point of tongue will apply this radial & linear directional force. Meaning in short that you will rotate on your axle center until you find equilibrium with the direction you are pulling.
If the distance you are pulling exceeds that of the triangle formed between the tongue and the two wheels of an axle, then you will drag the trailer for a bit (until the trailer rotation catches up). For the correct answer you may want to check the Math & Physics stackexchange sites.
Here's a quick example of something close to what I was thinking, this was done using constraints, and it is not updating the pivot point, so it looks kind of funny when you get away from it.
I do believe that the solution to this is using drivers, but I'm not totally sure, without spending the time to play with them.
When I get a moment, I will try to update this answer, as I am hopeful.