They do exist. The problem is that these seem to be semi-custom and rare builds, not mass produced and cheap units. Hard to believe that's so.
But, here:
Shuttle bus on the F450 platformand this. F550There are more but they seem to be rare and let's just say not a dime a dozen. Then of course there is the Super C which by definition uses a truck front but is considerable more expensive than regular C class.
which defeats the very point. If they made an affordable compact Super C, say 25-28' with a F450 front. The ones I found are 30+ and between 150K-200K.