Beginning approximately 2011-2013 (depending on manufacturer) new trucks have been coming out of the factory with bed/tailgate height taller and taller off the ground. There is no logical explanation or justification for this drastic change in design and specifications.
Example: Our 2003 Dodge 3500 (1 ton) single wheel measured 52 ½” at the highest point of the bed sides& tailgate. Our 2015 Chevrolet 2500 (3/4 ton) measures 58 ½” at the highest point of the bed sides and tailgate. This additional 6” in height, reduces the clearance from the tallest point of the sides & tailgate, to the bottom of the gooseneck frame rail on any older gooseneck trailer.
Trailer manufacturers began to rectify this problem, by building the gooseneck frame rails further off the ground (approximately 2011-2013 to present )- but not universally. If a trailer is leveled correctly (with the front of the empty trailer 2” higher off the ground than the rear of the trailer), there needs to be a minimum of 6” clearance, when the trailer is hooked to and carrying the tongue weight of the trailer, from the highest point of the bed sides & tailgate to the lowest point of the gooseneck frame rail.
The only two ways to make an older gooseneck fit under a newer truck are:
If the trailer has rubber torsion axles (not steel spring axles) then the axles can be removed and up to a 3” piece of channel steel can be welded between the axle box and the top of the reattached axles (Cost $600-$800 depending on how the torsion axles are attached to the trailer). This will gain up to 3” of clearance, safely, while maintaining correct balance. The negative is that this additional trailer height, off the ground, will make a step-up higher for loading and unloading or it will make a ramp steeper off the ground.
Removing the bed of the truck and replacing it with a flat bed, will negate this clearance issue entirely. The negative is that the utilitarian features of truck bed sides is lost.
Extending an adjustable gooseneck coupler will not solve this problem. Adjusting the coupler to make it longer will increase the clearance distance between the top of the bed/tail gate and the gooseneck frame, however, it will cause the trailer to no longer be level. When the coupler is extended, the front of the trailer will be significantly higher off the ground, than the rear of the trailer, putting dangerous excess weight on the rear axles and tires. This can cause tires to blow out and/or rear axles to bend (a blow out, especially when a trailer is loaded with livestock is terrifying and exceedingly dangerous). Often fenders are also destroyed during a tire blow out. Replacing a bent axle can cost between $600-$1200, not counting replacing any tires that may have been damaged by running on a bent axle.