There are two issues with the Rambox and a fiver.
Width is a problem. I don't have the Rambox, but with a utility bed I also have a narrow bed at 50 inches. Fortunately, since it is a custom bed with a lower height, the reduced width is OK because my extended pin box rides over the top of the bed rails, otherwise the back of the pin box would hit the sides of the bed. However, the height of the Ram bed precludes this solution, so you will have the risk of contact between the pin box and the sides of the truck bed. There is one solution available today and that would be the Sidewinder (aka Reese Revolution) pin box.
Found this comment on another forum; "Pulling a 5th Wheel with a RamBox?
I talked with a Ram Truck Rep who said Ram does not recommend the RamBox with a traditional 5th wheel hitch. Only option would be to convert the Bighorn to a gooseneck but Heartland strongly discourages that so I'll just stick with a regular bed."
Second issue is access to the storage boxes while hooked up. Since the fiver is wider than the truck, it will be hard to locate and retrieve items if the lid cannot be fully opened and you can't see inside the storage area.