Any discussion of percentage of discount on a used motor home is completely pointless. There are a hundred variables possible including major ones such as mileage, equipment level, condition, make and model, etc.
One can find units priced 50% more than true value to units priced at a bargain if one paid full asking price. There are two kinds of buyers. Those that insist on using the discount offered as an indicator of value and those who have done the research and actually know what the true current value is for a particular unit compared to what is available. Theoretical "book value" or some suggestion found on the Internet is not a substitute for comparison shopping.