I hear Koni is making available their class-A FSD shocks very soon for the E350/E450. They are soft when can be and go into heavy-duty mode as road conditions dictate. It is internal to the shock, changing instantaneously.
CLICK HERE to read about the technology.
CLICK HERE to watch a short video at an RV show. A Koni rep mentions the benefits on Class-A rigs.
I would hold out and get them.