Just traded in my old 5th wheel for a bigger/heavier one. I drive an '05 CCSB 4wd with Cooper Discover ATR's, 245-75-16. My GVWR on the old unit was 10k. This one is 15.5k. Tongue weight is up from 1650lbs to 1820lbs. I run the Coopers at the recommended 80psi and hover at 63-64mph. With the old trailer, truck felt solid. Just brought the new trailer home 4 days ago & it was very squishy in the rear. The trailer was empty & sticker weight is 11.5lbs dry. Thoughts? TIA. -John

air bags and better shocks would help. And pilates would help with the squishy rear end.

I did forget to mention, I do have airbags. The movement I am referring to is more left/right squish than up/down. Airbags are controlling the up/down pretty well.

it may be your tires. I am not familiar w/ the coopers, but that sounds like sidewall flex. you could try more air. I know you have said you have 80 psi, try 85 or 90. I know people are going to scream about that, but we have done this for many years on our truck cranes. we would air them up to 125-130. we had less blowouts because the tires didn't squirm around as much and also stayed cooler.

or move to a bigger e rated tire. a 285 in e rating would have a higher load capacity than a similar 245 e rated. might give you a little more room for error in running 80-90 psi in the tires.

it does sound like sidewall flex, like pecker said.

You have a 3/4 ton right? The GVWR is only 9,200 lbs. not 15.5. How much pin weight does the new trailer put on your truck? Bet you are way over the trucks specs.

Sorry, misread your post .The trailer GVWR is 15.5, not the truck. Still think the pin weight might be too much.

if the new trailer is longer behind the axles, there is more leverage for swinging the front end back and forth, and that swings the trucks rear end back and forth. Air all the tires to sidewall max, (truck and trailer), scale the truck and see if the toung weight is what they claim it is.
Also, you may be needing a rear sway bar on the truck..