I did mount my portabote on the side of my trailer. It also works well, and is very convenient to load and unload.
Just posting this as another method, and I'm not saying the underbelly method doesn't have a lot of merit...it does.
The link below on The Jayco forum includes photos, both inside and outside,and shows how to reinforce and then decoratively hide the stud reinforcements. You may have to log into or register on that forum to see the photos but you can read the narrative as a guest. Read through the two-page thread, as some folks asked questions and there are detailed steps later in the thread.
Did this two years ago, and have travelled lots of trips and miles with no problems. Also, total width of TT including the mount is just under the legal limit.
Portabote successfully mounted on side of trailer