I bought a similar level (not the same brand) and am still thinking on how to mount it. I was thinking that if I mounted it on the Sidewinder it would not be accurate unless you backed straight into your spot. If you had to back up and turn at the same time, the level would not be reading totally side to side but would also be starting to read some from front to back. It then wouldn't help you a whole lot.
I thought of mounting it directly on the front of the fifth wheel cap. I have a row of LED docking lights across the front near the bottom and there is room underneath those. But that area is not vertical and the level would be tilted. I wondered if that would be good. Also, I would have to drill a couple of holes in the cap, but I guess I could seal those with silicone around the screws.
My last thought was to make a couple of flat aluminum mounting brackets that attach to the bottom of the cap on the horizontal surface. The brackets would be long enough (three or four inches) to stick out the front and put the level on top of those. It would then be vertical and secure to the fifth wheel, showing a true level.
I am still thinking on this and haven't settled on any method yet. In the meantime, I am leveling side to side with a regular carpenter's level and have to get out to do that.
As far as your question about what is inside the kingpin box, I think it is pretty open in there. We had a bird build a nest in ours a couple of weeks ago and I had to clean it out. I could run my hand way in there and felt no obstructions.
Ron and Elizabeth
2014 Rockwood 8285-IKWS
2006 Ford F250 6.0 Diesel 4x4