It seems that the E350/450 chassis comes from the factory with an alignment that is NOT good for a motorhome application.
AND....in my experience, almost NO RV dealerships are equipped to handle the alignment, even though some will claim that they are.
So....before you spend a lot of money on throwing parts at it, find a truck service shop that can do a proper alignment. It did WONDERS for the stability on my C.
NOTE: Even though it had only ~30K miles on it, the first place I went tried to tell me that it needed close to $1000 in new parts in the front end before they could do an alignment. The second and third places I went said no such thing. Be careful of scams.