You can negotiate up to 15 to 17% off the MSRP of a new Class B. You are not going to come near 30% off. If you go to more than one dealer, you have more negotiating power, as the three dealers that we went to each came up with a different price for the same year, model, and options ordered from the factory and took the lowest of the three to the dealer we liked the best to negotiate down. We got to 17% off the MSRP. Class Bs are expensive and the dealers know that they can get what they ask - but none of them wanted actual MSRP, each was discounting to some degree.
The way to go lower is to look for a year old on the showroom floor model that you want and if they are interested in moving inventory you can do a little better (but not that much- one dealer wanted almost the same price as new, current year for a new B that he had sitting on his lot for two years.)