From my past experience somewhere around 30% is pretty good. RV's are hard to price due to the lack of invoice data found on the net. Dealers charge as much as a person is willing to pay. The auto industry is much easier to research, you can find invoice prices on them all over the place. We negotiated down to what amounted to about 35 to 36% on our last purchase. Where ever you feel comfortable with the price would be the best deal for you...
Remember that dealers set their own MSRP so the highest percentage off may not be the best price. One dealer may say MSRP is 40K and another may say 35K for the exact same unit.
Mine was roughly 40% off msrp but it was a current model year at the end of the year with a couple weeks of negotiations over the phone almost 300 miles away and I had to get it off the lot before the months end.
Being this close to the end I'd wait if I were you.
Just bought a 2016 Tracer Air270. Dealer seemed pretty motivated to sell (2017's were starting to come in) and I got it for a little more than 35% off their listed MSRP with minimal haggling. (We actually liked another camper better, but the dealer agreed to my price and it was too good to turn down).