The F-550 chassis is built heavier, at least 5000 pounds more capacity, possibly more with chassis options, but that far enough and it becomes F-650.
For the RV, it means they can build a bigger RV. It is better for those for whom bigger is better, with respect to the house.
I hope the RV on the F-550 is not 13,000 pounds heavier, that more than uses up the extra capacity, would be stretching the limits of a Class 7 chassis rating.
Engine size is not as relevant as other chassis parts. It is pretty common to de-rate engines to lower numbers, as the truck gets heavier, before stepping up to the next engine size. Particularly if they are starting out with advertised ratings for pickup applications, which are big numbers for limited duty cycles.
Tom Test
Itasca Spirit 29B