Winnebago does not usually build on chassis that have GVWR way over the vehicle weight, so if you find that one on a Ford, it will likely be on the E-350 rather than the E-450. GVWR will usually be within 2000-3000 pounds of empty weight to handle empty vehicle plus expected load.
I don't know the current E-series specs, to know exactly what the GCWR for 2013 model year E-350 will be, but it will be lower than that rating for the E-450. Last model year for which I have a brochure (2011), E-450 GCWR was 22,000, E-350 was 18,500 with V-10, or 10,050 to 13,000 with V-8, depending on axle.
Building the smaller lighter C's on a "one-ton" Class 3 chassis rather than a Class 4, a manufacturer can offer a C that has a better ride (springs loaded to "normal" load) and geared for better gas mileage. To this end, rental companies have been buying their smaller motorhomes on Express 3500 or E-350 with V-8 rather than V-10. Of course, they don't allow their customers to tow, so they don't need excess towing capacity.
On a C that small, I would rather have the Chevy, with the 6.0 V-8. I would prefer the better gas mileage and cab ergonomics. But if I were buying new, that small, I think I would be looking for something on Sprinter, it's even better than the Chevy and will tow slightly more. But Sprinter C's are not likely a choice for you, if you are looking at entry level for the lower price.
Are you planning to tow something like 10,000 pounds, which is E-450 maximum tow rating? Or load 5000 pounds of cargo into a 9000 pound motorhome? I think there are a few smaller manufacturers who will custom build a small C on the E-450 chassis, or use a "one size fits all" approach to managing their chassis inventory.
Since you can get into that particular coach, check the weight sticker in the closet. See if the weight of coach, tanks, and people are where you want them to be, relative to the GVWR on the Express 3500. If you would like to have more carrying capacity, seek out a manufacturer who builds a similar floorplan on a heavier chassis.
Tom Test
Itasca Spirit 29B