I would ordinarily agree that blanket % talk is not helpful, but the problem is pricing is allllll over the place. Using percentages at least gets you in a ballpark of reasonable pricing when they are so inflated. If I didnt at least think about a %off MSRP when i started I may have walked out thinking paying 30k for a 34k trailer was a great deal... when in actuality most people who list what they paid were another couple grand short of that. The apples to apples thing is tough too because some dealers are super shady with what is different between units. I dealt with one that said, "well, are you really comparing apples to apples.... ours has LED lights, a 20' awning... etc" I told him after he stopped talking that he was listing "options" that both he and I knew were standard on the unit we were discussing. Needless to say I did not do business with them.
In the end I treated the % thing as a starting point and then accepted pricing differences based on things you mention like current market, dealer stock, proximity, etc.