The Bilstein versus Koni FSD owners' opinions/impressions probably have a lot to do with how big of a Class C they're using them on - both side-surface-area-wise, overall-weight-wise, and front/rear-weight-distribution-wise.
Note that the Koni FSD shock part numbers are IDENTICAL whether you're buying them for a Ford E350 or a Ford E450 ... and on top of that ... regardless of how much weight, or wind force, or road curve centrifical force - the coach is going to expose either chassis to.
Of course any one particular shock can't handle all of the Class C coach/chassis combinations using any one single damping rate (Bilstein) or any one set of variable damping rates (Koni FSD).
For larger/heavier Class C RV's on the E450 chassis, most likely the HD Bilsteins would do better most of the time in most situations ... and for Class C RV's on the E350 chassis, most likely the HD Bilsteins would again do better most of the time in most situations. In both of these scenarios the suspension system needs shock help for controlling of a relatively higher weight/force to chassis ratio.
My particular situation was different - a smaller/lighter Class C RV on the E450 chassis. For this setup a full time heavy duty, or a full time soft, shock would not be the way to go. I DID NOT need an "HD" shock on roadway cracks (vertical forces) to add to the rear leaf spring stiffness. At the same time I DID NOT need a "soft" shock for side-force control (the more stiffness the better for that). Hence, a shock with more than one damping rate was better.
It boils down to how much motorhome does it's suspension have to control, relatively speaking. What Koni really needs to consider is different FSD shock designs/part nos. for the E350 chassis versus the E450 chassis ... maybe even four(4) different shock designs/part nos. for: Lightly and heavily loaded E350 chassis, and lightly and heavily loaded E450 chassis.
Of course the ultimate is an infinitely-variable, electronically-controlled $$$ shock system like I believe that some ultra-high performace racing cars use. Koni's FSD shocks are a step in this direction for some RV setups.
2005 E450 Itasca 24V Class C