Perhaps in the old days, prior to the banking crisis, RV industry shakeup, and gas price increases, you might have been able to plan on some arbitrary number like 25-35% off MSRP. But now you need to do more research on the model you want, and the dealer who has the one you want.
Obviously, any dealer will discount a unit he has had in inventory for quite a while, more than one that was ordered and just arrived. Or if he has several of the same model. Have you checked to see what the nationwide inventory on that model is via internet search? Knowing what other dealers are marking their inventory at, gives you some leverage on your local dealer.
The more expensive a unit is, the more mark-up% may be available for a discount. Do you have a trade-in, or is this a straight cash deal? Is the dealer going to do the financing where he might get a bit of a fee for arranging it, or are you doing the financing yourself?
And is the market for RV's in your area consistant throughout the year, or seasonal? I would think that California is fairly uniform, where Texas or Minnesota would have definate low points in sales activity.
And is the unit you want, new but last year's model, with the new ones about to arrive on the scene? All factors to consider.
That being said, we bought our unit, new, but end of the model year, a sort of a rare model, we had a 7 year old RV to trade in, and it was at a low point for the RV industry. Its hard to figure in the exact discount with all the factors, but I estimate it was around 25%. Could we have gotten more, possibly, but a cash deal talks much louder than one that includes a trade-in.
Final advice, don't be afraid to make a reasonable offer, and then walk away, you can always up the offer later when they call you back. Sometimes it is better not to get the deal you think you want, because a better one is around the corner.