Unfortunately, this is a problem for many rvers who have older 5th wheels, & try to mate them up with newer trucks. The newer rvs are designed to match up with the newer trucks, but the older rvs come up short. I have a 2008 New Horizons, & originally towed it with a 2008 F450 diesel dually. It towed perfectly level with about 7" of bed clearance. I then upgraded to a 2017 F350 with the higher bed walls. I had to add an adapter to the base of my Hensley Trailer Saver hitch to raise the hitch height, so that I could get a minimum 6" clearance. But the 5th wheel now rides about 2" high in the front. That's not a huge problem, but I'm guessing it has some affect on tire wear & tire weight capacities on the dual axles of the rv, with additional weight shifting to the rear axle.
You have to be very careful, towing a 5th wheel, even with 7" of clearance. Transitioning over uneven ground, especially during a turn, like entering driveways, can quickly cause one side to make contact with the bed rail & damage the truck or 5er. In the 200K miles we've travelled in the last 12 years, my 5er has made contact with the bed rail about 5 times. Four of the times were slow, methodical turns where I was watching the spacing & the contact was very light. The fifth one caught me off guard & caused a slight crinkle in the bed rail, barely noticeable. If you're going to have to live with 6" of clearance, you'll have to be very vigilant about watching your turns into driveways.
