The storage bay on the 320RS is pretty decent size, but I still needed some more room to take all the stuff that I need, especially when on trips of a month or more.
After much surfing on Amazon, I decided on a particular tool chest to mount on the rear bumper. It is made by DeeZee here in the USA, and is about the right size to fit the area that I wanted to put it in. I wanted the chest to fit between the ladder and the license plate. I also didn't want it to over hang more than 6". The DeeZee box also has a recessed lid, so when I mount it on the bumper, it will be able to fit tight against the back of the RV and still have the top open easily.
Amazon.com: Dee Zee DZ8546SB HARDware Series Steel Utility Chest Tool Box: Automotive
I knew that the single 4" bumper on the rig would not be sufficient to hold the tool box firmly over rough road surfaces. I went out this morning to the welding shop here in town. I had another rail installed between the existing bumper supports. From the time I pulled in until I pulled out was 30 minutes. The cost was $40 including the iron.
I came home started the install on the tool box. After painting the new iron support, I had decided I wanted a bit more support on the front edge of the new tool box. Even though it would only over hang the bumper by 4", I wanted to add some support to the front edge. I picked up 3 4" "L" brackets at the local hardware store and riveted them to the front of the existing bumper.
I attached the new tool box to the existing frame rail, bumper and "L" supports. The result was rock solid mount and another great storage solution for the usual camping stuff that I take with me on the road.