The biggest challenge for dealers, is the possible defects lurking in the trades they accept ("buyer beware" applies to dealers too). Couple this with buyers that expect used units to be brand new and it's easy to understand low prices paid for trades. Dealers sometimes incur significant expense repairing trades, even to the point where they are forced to cut their losses and simply sell the unit to a wholesaler just to get it off their lot.
Just like any retail business, rv dealers must buy at wholesale and sell at retail. If they don't, their doors will quickly close . . . and many do each year. One mistake I read all the time, is how consumers think dealers should treat the profits from the trade and the new sale as one. Well, they are truly two separate inventory items and each must earn a profit on their own.
So, in the end, don't expect even low retail when trading. Anything at, or above, that number will be recovered on a higher price you pay for your new unit.