this certainly seems to be an issue. I have a 2014 Ram 3500 completely stock and my bed rails are 59.5 from the ground. I am looking to order a toy hauler here real soon and the xlr I liked is right at 60.5 from the ground to the underneath of the overhang....and this is a 16500 GVWR TH with axles already flipped and 16 inch tires.
I hooked up to a couple smaller fifth wheels and they were a good bit nose high and I didnt have clearance. Hooked up to a Grand Designs unit that was around 12500-13k dry and it settled about 2-3 inches and was about perfectly level with 7 inches bed rail clearance.
If you have a newer 4x4 truck, you are most likely going to need a newer 5th that has enough pin weight to settle the truck a bit...the lighter 5ths are going to be tough to get close to level imo....