New 5th wheel doesn't tow well

What you are experiencing is 'chucking'. It is part and parcel of the fifth wheel towing experience unless you use a hitch that provides some 'give'. Most of the hitches that help mitigate chucking use some type of small air bag as part of the hitch or the fifth's pin assembly. You might google 'fifth wheel chucking' or 'what is fifth wheel chucking' for a good explanation and read all about it. Hensley makes a hitch that does a great job eliminating chucking, a little pricy, but it works. MorRide makes a pin assembly that helps mitigate chucking. Good luck.