I bought a Sondar backup sensor years ago for my 99 Dodge Dually, and the sensor went with the truck when I sold it. It was indispensable for backing up an 18ft big truck. One nice advantage is that no holes were necessary, it came with brackets designed to mount underneath the bumper using the existing bumper mount bolts.
It was a two sensor system with readout in ft and inches from nearest obstacle, but with LED's, it also told you were the obstacle was located in the rear. (left, center right).
Now that I have a new truck, I would like to install another one, but I can't find the Sondar brand anymore, nor can I find any that come with brackets for trucks to be mounted under the bumper.