I own a gas Class B, but I think in the end the costs of a gas and diesel will come out about the same.
If you assume diesel costs on average 10% more than gas and the MPG is about double for a diesel, then the money saved on fuel alone should more than cover the costs of the "A" and "B" services.
For instance, if gas is $3.50/gal and you get 15 MPG, diesel is $3.85/gal and you get 30 MPG you should be about $1,000 ahead at 10,000 miles and $2,100 ahead at 20,000 miles with diesel. (Unless I screwed up the math that is!)