Welcome to the forum!
The only way to know you have the optimum tire pressure is to get your rig weighed... at least by each axle although four corner weights are better... and then use the tire mfgr's pressure chart to determine the pressures you need.
When I picked up my new 40DP I had the same experience. I knew I needed to get it weighed but hadn't yet done it when we took off on our maiden voyage from Sacramento to Phoenix. I had the tires inflated to 110 # since that was listed on the placard in the coach. The ride was so harsh that I was afraid I had made a mistake in my purchase.
Once I got the coach weighed, I could reduce the pressures to 90# and it made all the difference in the world.
Rick, Nancy, Peanut & Lola our Westie Dogs & Bailey the Sheltie.
2007 Itasca Ellipse 40FD