I have worked with ultrasonic range finders in some of my projects back in the day. The fun thing with them is the sound pulse they emit is conical (radiates in a cone shape from the speaker element) and it can generate a feedback wave from the ground. Also, they like the target to be a solid object. If you are looking to target the trailers, you might have a problem because they are mostly air.
Here are some things to look out for:
Draw the cone in a 3D CAD model and make sure nothing is inside the cone before your intended target.
The sensor has a minimum effective distance (somewhere between 4"-6" usually - this should be on the spec sheet) so make sure your target never gets closer than the minimum distance.
Ignore values higher than some optimum distance. Eg: If you intend to start acquiring your target at 20", ignore values greater than 25" because they could be junk reflections from objects other than your target.