It costs money to carry an inventory. You can't blame a retailer for identifying the parts most likely to sell and then stocking those with the highest margin. With analytics today, it is quite easy to do that using statistical methods based on historical parts sales. Also, don't discount that the manufacturer has set a high wholesale price to the dealer to offset a lower RV price-- much like HP will sell you a printer at cost + 5% and then hope to make it up with a much higher margin on the ink ($35 per cartridge). Here, the manufacturer might be able to used a standardized, commodity part, but if they design and use a proprietary part, they can charge much more for it. To make a fully informed RV purchase, one really does need to look at the aftermarket for parts and service. For example, I love my Winnebago on the MB Sprinter chassis, but when engine works needs to be done, compared to the Ford gasoline engines, I am going to be in a world of hurt.