RV's weigh a lot requiring stiff springs. Interstate highways are rough from weather and concrete sections are cupped due to pounding by big rigs. Reducing tire pressure within safe limits can help. A few have removed rear spring leafs or had stock springs modified for softer ride. I have always tried to keep speed down and find the smoothest lane taking frequent rest stops to reduce road strain. I have had tire failures on really hot days while taking a pounding from rough cupped highways. Secondary asphalt paved roads can be a lot smoother.