Gulf Stream put the Yellowstone body on 3 different chassis, Ford, Chevy and International, some of the Fords were E 350 or E 450. Here's a link to the brochure for the 2007 GS Yellowstone, it might contain the info you seek.
http://www.gulfstreamcoach.com/towab...ature-0108.pdf
Also, you might contact Gulf Stream with you VIN, they might be able to tell you, or have someone in SD at the storage location open it up and look at the Federal sticker for the weights.