Rules of Thumb for Axle Weights:
33% or More of Total Scale Weight on Front Axle, or...
Front Axle Loaded to at least 75% of its Rating...
These can work out to about the same thing
And of course neither axle overloaded. These numbers should be on a label in door frame of the cab.
An E350 under a mid-20-ft coach is very close to its max capacity. I don't want to be the Kill-Joy Nay-Sayer, but I'd think twice about carrying extra weight in or slung under the cabover. They're an awkward structure and subject to stress and strain on the road. More weight just increases the forces and threat of developing leaks.
The good news... If you have a reasonable weight distribution, there are a few things that can be done to improve steering and tracking. The first and easiest is to be sure the front tire pressure is no higher than what the axle weight requires.