5th wheel

I think 4 inches is sufficient. I have about 6 inches with mine and I go a lot of offroad off camber turns without issue but I also have an extremely shallow pin depth and that helps. But when I was pulling a bullrack with standard pin depth I didn't really have any issue either.

Biggest thing to watch for is hooking up to the trailer. If trailer is a little low and your fifth wheel is tonfar forward you will crunch those fenders in no time. I have the back of my half fenders sitting a little lower in the back and set the fifth wheel in a position that the trailer will start lifting before it can contact the fenders

 

On my other truck, that I pulled tanks with, I took it to Brenner, used to have one in west Memphis, no longer, but they had a welder make a riser, he hooked it all up, he was certified for all kinds of welds, he showed where he had taken a couple of cam locks and made knives out of them, and had a machete made from a wrecked tankers dome lid.

I’d say run it by a brenner or grace trailer that’s certified for tankers, they’ll build and you won’t be able to tell it from factory.

 

You can see in this pic the trailer sits pretty high. The king pin plate bolts under the trailer making it pretty high, where many trailers the plate is flush with the trailer. I've pulled hopper and livestock trailers that sit pretty close to fenders without issue. You can also see in that pic that my king pin is pretty close to the front of the trailer (shallow pin setting).
I'd guess thats around 6 inches or touch more.
you can see in this last picture hooked to a grain trailer that sits flush with the fifth wheel theres not much clearance between fenders but not really an issue and you can see the angle I have the fenders mounted. Might not look the best with them lower in the back but it really saves them from damage I think

 

And depending on how your fifth wheel slider is bolted to your truck it could be very easy to add a spacer and lift it an inch or something

 

Looking at my last picture hooked to the grain trailer and seeing how much the fenders drop in the rear and thinking if they sat flat i would definitely want more clearance. There would be contact way to easy if they were indeed level to the frame