My wife and I have recently0 purchased a used 5th wheel to see how we like it and to take a break from vacationing in our LQ horse trailer with our horses. What I am trying to figure out is why the 5th wheel tows so much harsher. Towing the gooseneck LQ horse trailer you literally do not even know it is there. The 5th wheel on the other hand lurches front to back. I see the posts for Demco ect but wonder why you need to do that in the first place? The 5th wheel has springs and shocks.
Some fifth wheel RV trailers are designed better than others and tow/ride less harsh than others. It varies with axle spacing to the kingpin, overall balance, and trailer weight and suspension components used.
I have experienced and agree with your comparison between some fifth wheel trailers and gooseneck horse trailers. I've pulled a few of those very expensive all aluminum ones out of plants in or around OKC. They did, indeed, tow like a dream. I think it is probably due to the rubber torsion axles they use instead of steel leaf springs that are the design first used on horse drawn wagons. It may also be related to balance point and percentage of trailer weight shifted to the rear axle of the tow vehicle. Fifth wheel RV trailers are balanced so that 75-80% of gross weight is on the trailer's axles and 20-25% is on the king pin. I don't know the figures or percentages but horse trailer axles appear to be set back farther and probably transfer more weight to the tow vehicle.
As you know, I am a strong believer in the performance advantage of Demco Glide-Ride kingpin assemblies. They will absolutely eliminate the "chucking" motion so common to fifth wheel trailer towing.
But beyond the specialized king pin assembly, it makes a difference what brand of fifth wheel hitch you are using. This comment will cause at least a half dozen TDR members to jump up and post to defend their brand of hitch but Reese, the most commonly sold and used hitch, the oldest in the industry and, in my opinion, best fiver hitch on the market, is also the tightest and allows the least amount of slack when the kingpin is coupled. Some of the other brands, relative newcomers to the market, are less expensive to buy and work fine and reliably but use different latch mechanisms and allow a lot more slack between kingpin and collar. I've known several fellow RVers who bought a cheaper brand initially to save money and later dumped their hitch and bought a Reese.
If the annoying movement you feel is a hard jerk on start and stop, it may be the hitch. If it is a continuous chucking felt at towing speeds, a Demco is the answer.