As much as I'd rather trust people, don't believe anything a salesperson tells you.
The RV industry does not have the same 'big gov.' laws on them as does the consumer auto industry, so there really isn't a factual sheet like for cars. A dealer does know how much a unit cost them, but it'll vary depending on transport costs, etc. Each one does have a cost though, a sales person must make a commission, and a dealership must make a profit, so don't expect to get one at a loss (UNLESS they have some other business related reason to get rid of it right away!).
It all really boils down to negotiating and what both sides will be comfortable with. I've purchased seven RV/trailers over the years and have realized that what I personally want is reasonable/good(not super) value and great quality & reliability. I'll also get up and leave if someone says they've got to 'talk to their manager', hands me off to someone else who tries to jack the price up with extras. or makes me wait more than ten minutes for some bs reason.
Finally, I always, ALWAYS, insist on performing as thorough an inspection as I can. This includes slides in and out, water in tanks & pressurized with no leaks, everything operating, etc. If they tell me they won't let me do it for some lame reason then I won't buy.