Reverse Backing Systems has some really nice models that i personally like. The one i have has four sensors which gives much more coverage in the back.
Also, the truck model (the one i have) works up to 10 feet away (instead of the ~4 feet on the other ones). It only alerts you within 5 feet witht he beeping.
I am sure you can include a toggle switch to turn the sensors off when you are backing up your trailer.
The model i have is here:
http://www.reversebackingsystems.com/ETA-S.html