"This reminds me of a subtle point. A Class C coach (like tpi's and ours) that lightly loads an E450 chassis may need less use of the Tow/Haul mode's off-throttle downshifting advantages because normal engine braking is stronger. Since an E450's rear differential has a lower ratio (4:56) than the E350's, the E450 motorhome's overall engine to rear wheel gearing ratio is lower. Hence the engine will hold it back better when off the gas coming up to stops or when off the gas going down grades.
This is another reason I advocate use of the E450 chassis on smaller Class C rigs that, from a pure weight perspective, could get by on only the E350 chassis. "
The performance of the E-350 is hardly a matter of just "getting by". The reason the E-450's have the lower rear end, 4:56 vs the 4:10's of the RE-350, is all of the extra weight. Typically the 450's are found on larger to much larger motorhomes which necessitates the lower gearing to handle the load. On a Class C under 25' a 450 is overkill but if you want less mileage and a rougher ride by all means go for it, but there is little if anything to be "gained".
As always.... opinions and YMMV.
:C