We improved the braking performance of our 2007 E350 PC-2350 by selecting the a Unified tow braking system for our Jeep Liberty. When towing the Liberty and the driver-adjustable sensitivity is set to the top limit, it actually reduces braking distance as compared to towing nothing at all.
I realize it's not the type of answer you were seeking, but it is the only thing I know of that directly addresses your inquiry.
Here is the braking distance chart off the manufacture website. The actual numbers would not apply given they are based off a different motor home, but the relationship does seem to be accurate based on our personal experience with the system.
Click Here To Read The Chart Better: http://www.usgear.cc/utbstop.htm
If you had rear drum brakes (which you do not), I would have advised you adjust the rear brake shoes yourself. If adjusted properly, the rear drum brakes will do much more for you than what they typically offer. But even at their best, I think rear disk brakes out-perform rear drums any day.
Other influential factors are things like traveling lighter, traveling without any on-board water or sewage for example. But then that defeats the idea of having a motor home in the first place.
Suspension upgrades would indirectly help in reducing stopping distance. A well planted rig will utilize it's current brakes better.
I just don't know of any brake upgrade available.
Your 2007 E350, the brakes all around are a bit smaller than on a 2008 or newer chassis. The quick indicator of bigger or smaller brakes is.....
If your E350 or E450 has the latest industrial looking front grille styling, than you have the bigger brakes. All rotors, caliper pistons, and pads are a little larger. Not much larger, but surely a welcome. I think the rotors are no longer integrated as part of the hub. They are more like a car is setup where the rotor slides over the lugs and hub, held in place by the wheel & lug nuts.
The 2009s also got an interior face-lift, an improvement in another category, with a glove box and two-tier door pockets, and a more practical dash board & center console. Things like the 12V sockets are in better places to be out of the driver's way. When my wife plugs into 12V, I can't use my gear shifter unless I hold it a certian way. It seems a safety hazzard.
Sorry I got off-topic.
It would be interesting to know if you could install 2008-2013 brakes on a pre-2008 chassis.