What one "gains" with the E450 chassis over the E350 chassis - with an RV coach's raw weight that doesn't require it - is (at least for my model year):
1) Larger brake swept areas.
2) Thicker frame steel.
3) Increased transmission fluid volume.
4) Hydraulically assisted power brakes instead of vacuum assisted.
5) Wider rear stance.
6) Lower rear differential ratio.
All of these differences provide long term advantages and are well worth the extra dollars we had to pay for the E450 chassis under a small Class C. I advocate "buying a heavy duty vehicle and using it in a light duty way" as the best way to help ensure safety and long term reliability with lesser maintenance.
There are ways around the less mileage and rougher ride of the E450 chassis, but very few affordable ways around having a lighter duty chassis to begin with.