http://www.amazon.com/Dodge-Sprinter.../dp/B0017RXXIG
Caveat— In order to account for rooftop equipment, the fit will be necessarily loose. If even the slightest amount of dirt or debris gets under the cover, the cover flapping in the wind will cause said dirt to work like sandpaper on the finish of your Interstate. The cover will also trap condensation, unless you can find one made of Gore-Tex— which I've never seen for any vehicle, let alone a Sprinter.
A good wash-and-wax before putting your Airstream Interstate away for the winter, along with a windshield/front window cover to keep UV sunlight off your dashboard, should do a better job of protecting your investment than will the cover you're looking to buy.