To get an E-series to ride comfy, you have to load the chassis to capacity, both the front axle and rear axle. Unfortunately it is very hard to load the front axle to capacity due to the teeter-totter effect. Pending the length versus the wheel base, the front axle often runs lighter than it should, especially B+s without the huge bed above, and so the ride can be harsh, surely more than in a pickup truck.
The E-series made after 2007 can be worse because Ford increased the front axle capacity with stiffer springs and such to increase the over-all GVWR. But many motor homes don't carry so much load up front, hence they have a rougher ride. I would imagine that you could get lighter-duty springs installed up front to remedy the situation, but most people wouldn't consider doing something like that.
