Oh there is plenty of clearance of the bottom or the 5th wheel nose to rails, it is the brackets the pin box is mounted too that is the issueI would be careful lowering your hitch as you want to maintain so much bed clearance. I have about 7" of clearance but I also do some extreme camping. I think 5" is a reasonable amount. I also flipped the axles on my 5er to get a better level rig as my trailer also sat like yours originally. It also dragged going into some of the areas we go.
And I was thinking about slightly bigger tires for the truck too!What about those low rider, rubber band tires? I see lots of them around here on lower trucks. Think it is a Mexican thing but you are closer than I am.
You could also get 10-12" inch rims and tires. Just don't run over dead things on the road unless you want to grind up some road sausage.
This is one of those deals like "Who's on first, What's on second, and I don't knows on third"