We have a 24 foot Class C on the E450 chassis with Michelin Load Range E tires on it (maximum of 80 lbs. pressure - as printed on the sidewalls).
I have used 75-80 lbs. of pressure in the rear tires and 60-65 lbs. of pressure in the front tires for years, and it has always handled "like a van". Too much pressure in the front tires can cause the motorhome to feel like it's wandering when going down the road ... which wears you out steering it. 60-65 lbs. in the front for us has not caused this affect.
I like fairly high pressure in all the tires (without affecting steering) regardless of going over what weight charts might recommend - so as to keep sidewall flexing and the resulting heating of the tire material to a minimum for long tire life.
However, maximum or near-maximum tire pressure in the rear tires does make for a stiffer ride in the rear on an under-loaded E350 or under-loaded E450 Ford Class C chassis. To help with this I've replaced the stock rear shocks with specific after-market shock absorbers that were not available until several years after we had bought our motorhome.
2005 E450 Itasca 24V Class C