You own a Winnebago/Itasca Navion. Winnebago does not make B+'s. They make Class A, Class B, and Class C motorhomes. When I look at information for your MH at the Itasca site they show it as a Diesel, Class C.
Class A... Built from the ground up by the manufacturer.
Class B... Built by customizing/gutting a stock van delivered by Chevy, Ford, Dodge, etc.
Class C... Built by starting with the cab/engine components delivered by Chevy, Ford, Dodge, etc. but building everything else.
A B+ is simply a streamlined class C. B+ was created by marketing types because some thought a Class C had to have a bed over the cab look. Besides B+ sounds better than a C-.