Do your trailer axles have shock absorbers? Some manufactures do not include them - or put them on one axle, not both. Many times they are worn out, and not that good of quality.
Is your hitch mounted with the center 1" forward of the center of the truck axle? If mounted behind the axle, it can cause bad handling on the truck and possibly trailer problems as well.
How much weight is on the hitch? Check the truck front and rear axle weights, then hitch up and check the truck again, then the whole rig, and let us know what you found. Also let us know if the water tank was 1/2 or 1/4 or full.
Most fifth wheels are pretty stable, so suprised to hear this.
Good luck,
Fred.
