Carrinb, I stand corrected as to the towing capacity. You're right I was thinking of the hitch capacity which in fact limits most of the E-450's motorhomes out there to 5,000#. "If" you can upgrade the hitch capacity, which is not always practical or even possible on an existing coach, the greater capacity of the E-450 will support out to about 7,500#.
I stand by my assertion that the 450 is not now and never has been 6" wider than a comparable (same year chassis), 350. On the Ford website they have specs of the 350/450 side by side and the differences are minimal in virtually every category.
Going from 3:73's to 4:56's and improving your mileage is a good trick but running in OD all or most of the time will be hard on your transmission under a load. My Scan Gauge shows the rise in trans fluid temperature on even fairly low and all steep (long) grades. Engaging Tow Haul (or turning off OD if you don't have TH) is the best way to reduce those temps quickly and efficiently. When I do the trans temp typically drops 12 to 15 degrees in just a couple of miles with a negligible impact on mileage.
:C