That's a dual band VHF/UHF (2m/440Mhz) magmount. I used it cause it was lying around unused and paid for!
It's never used when the truck is in front of it. It's only used after the rig is set up and then usually only to communicate with either the truck or a HT that has wandered off someplace. So to answer your question, it works pretty well, remembering the 5th wheel nose is made largely of fiberglass and the pinbox is plenty of ground plane at these frequencies. It would probably work better if mounted higher and level, but it's out of the way and no trouble with limbs and such as it is.
You might like this thread on our sister site: http://www.openroadsradio.net/forums...read.php?t=676