I think if i were to mount this, I would drill a couple of holes on the bottom of the box and through a metal plate of some kind that would lay flat on the bottom, inside the box. Then run flat headed bolts through it and around the frame, through another plate on the bottom of the frame so that it grabs and hangs on. A pinch mount sort of thing, that way it can be removed if needed and also you dont risk weakening the tongue or a rail of the tongue by drilling holes in it. Might be easy enough to just load it in the front box....did you get a full slide through box in the front with your model?
Where we've been:
2000 Rockwood Roo M-18 Hybrid
2006 Dodge Ram 2500 Auto 5.9 Diesel
Gone but not forgotten -1986 Rockwood 1003 PUP