Hi chiefward,
Everyone is offering great insight, and here is mine.
Any class C motorhome is going to have a rough ride when these two conditions exist.
1) Tire pressure is too much for the actual load on them. Fronts versus rears are treated independently.
2) The max load rating of the chassis far exceeds the actual load on it.
You mentioned a U-Haul box truck. Load the box truck to it's load capacity and the ride smooths out. The same applies here.
My rig is built on a 2007 E350 chassis rated at 11,500 pounds. The actual weight of my rig when on trips with people, gear, food, full fuel and water tanks, etc, etc, is at it's sweet spot of around 11,000 pounds. That leaves roughly 500 pounds of margin.
That along with proper tire pressure, our ride is very comfortable. The ride is not as soft as our family sedan, but is quite acceptable. It also mentally helped to improve our road trip experience when addressing all the little rattles around the house.
So.......
If you have a rig built on an E450 chassis with a 14,500 pound rating, but the actual weight is like mine at 11,000 pounds, you have 3500 pounds of weight margin. That extra margin does not seem to be massive, but it will knock your teeth loose, not to mention the damage being done to the house and contents. That is partially the reason for water getting inside motor homes. The caulked seam work and general construction can't handle the constant abuse. That is why some people like myself stress considering a better quality constructed rig. That would be the better question......how to identify "better" of which I have a very long winded reply for that question.