I don't consider that a huge mark-up (under 40%) and if the dealer sells at his cost he won't be around very long. And you may not be happy either, since he has no gross margin to cover the necessary expenses of delivering the motorhome (dealer prep, walk-though for you, etc.), let alone a commission for the salesman and rent on his store.
Mark-ups in retail businesses can vary from under 10% (grocery stores) to more than 200% (jewelry and electronics). The amounts reflect a variety of factors, including sales volume/turnover, shrinkage/loss/returns, inventory finance costs, customer delivery expense, etc. 40% isn't excessive for a big ticket item with low turnover and a lot of delivery expense.