Welcome to the forum.
One thing I found a few years ago when I bought my Class C was the floor plan size will generally dictate the chassis that is used and there in nothing a consumer can do to influence the decision other than go with custom built.
I also found that most manufactures generally go with the E350 below 26 or 28 feet and then move up to the E450 at sizes over 28 feet.
Also, of note, unless you are buying a Super C you will be on the E series chassis and not the F series. There is a difference.
US Army (Ret)
2015 Thor Challenger 37ND
FMCA - F432054