GOOD information here; http://www.blueridgetrailer.com/blog/gooseneck-trailer-clearance-truck-bed/
DON’T: Buy a used trailer without testing your truck’s suspension
Just as trucks are getting built taller and taller, they’re also getting softer and softer suspensions. This can be either a blessing or a curse when you go to hitch up a heavy gooseneck trailer.
In some cases, we’ll see customers drop a gooseneck on their truck, and the suspension will simply be too soft to safely support the trailer.
Other times, the suspension will provide just enough give to create the minimum six inches of clearance between the trailer and truck bed, while still supporting the trailer adequately.
It’s nearly impossible to determine what the case will be for your truck and a given trailer without hitching it up and taking a look. So don’t buy a trailer without testing it out first!