I have a regular tool box in mine. It does not sit loww and is 6 inches above the rails. I have never had an issue with it even close to 5'er. I also have a heartland that has the 88 degree turn radius. But even without that I still never came close to the camper. I never had to jack it around that far cause I will pull up before I put all that pressure on the axels.
I had a box in the front, but had to make room for my HCCR Steps. I went with a rear toolbox from here. Although it is a bit awkward to use when hooked up, I enjoy having the tailgate as a working surface when unhooked.
If you use a rear one, you can't just be below the rails, you have to account for the drop and swing of your pinbox. In my case I had to go with a 16" high instead of the 19" high.
The Idea behind this type of tool box is you don't need a tailgate.
It would be much easier to use if you removed your tail gate.
At first, I thought the same thing and was sorry I had already bought the tailgate. But as it turns out, I cannot organize all my tools into those little tunnels and reach in and get just what I want. Many things are just lying in there, but I also have a couple of small toolboxes that I pull out and sit on the tailgate to look for what I want. I also use the tailgate surface as a working area. The V cut out of the gate allows you to move your body close enough to the toolbox to not get a backache.
I too use my tailgate as a work bench when out traveling. Plus my rear view camera is mounted in the tailgate. Wish it was in the top back of the truck cab but it is not, though sure handy when backing up to my boat hitch.
That web site Fred42 mentioned has several that look good for rear mounting. May have to give them a call.
Formerly of Colorado and Alaska
2016 Fleetwood Flair 31 B Class A w/bunks http://www.pajbcooper.com web site
Alaska-Colorado and other Trips posted
"Without challenge, adventure is impossible".