The only MSRP I would trust is the one published on this forum. When we were looking for our trailer, I found cases where the MSRP list price was inflated above HL published MSRP. I ended up not trusting those dealers to give me a fair price.
The percentage off has many factors: popularity of the model, time of year, time trailer spent on lot, new model year or end of model year... I believe the only way to know what the market price for the model is to compare all prices for that model using the web. That should be your starting point before you negotiate your offer. I found out that in Denver, there was little competition, so trailers in Salt Lake City were cheaper - go figure.