Ron,
You might want to keep in mind the size of the Class C that posting owners are happy with, or not happy with, regarding FSD shocks on (at least the rear of) their motorhome.
A heavy Class C has it's weight to pretty much negate the effect of single action stiff shocks having their stiffness added to that of the rear leaf springs ... so they would think that their single action stiff shocks were just fine because the stiff shocks would of course be helping with the most important-to-them thing ... body roll -> as does the stiff (low frequency) mode of the FSD shocks do with respect to body roll.
I'll bet that FSD shocks in the rear of your small Class C will definitely help to reduce any pounding that you may be experiencing from the rear leaf springs. HOWEVER, your E350 does have less stiff leaf springs in the rear than an E450 does, so "maybe" you would not notice much help from their high frequency "light shock damping" mode on highway cracks and potholes.
I'm not really sure what help FSD shocks would be on the front coil springs of a small Class C on either an E350 chassis or an E450 chassis. My mechanic recommended that I contact Henderson's Line-Up in Grants Pass, Oregon to get advice on what help FSD shocks may or may not provide to the front coil springs on the Ford E-Series ... before I decide what to do for new shocks on the front of our E450 24 foot Class C.
