Like someone else mentioned, get online and find your model at as many dealers as possible and get an idea of what the MSRP really is. I just purchased a couple weeks ago and when I mentioned that I knew the dealers set the MSRP the salesman freaked out saying there was no way. So, do they? Who knows but nothing would surprise me.
Once you get a good average for MSRP you should be able to pay somewhere between 60-80% of that. The wide range depends on brand/manufacturer, model year and location. Remember there is a lot of shipping charges for trailers. Especially further away you are from where they are made (usually the mid-west but not always).
For reference I just paid 31K on a 5th wheel with an MSRP of 47K. I was super happy... dealer was desperate with it being the end of a winter month and wanting to move something.