I cannot speak to the 350 specifically (we have a 380TH), but I'd expect to negotiate to around 30% off MSRP, so somewhere in the low-mid 60's. With that said, since you're upgrading, I would not get in a big hurry and do your research. COme up with what you're willing to pay and stick to it. These guys will make plenty regardless of the discount. If the local RV dealer won't deal, it might be worth looking out of your area.
My F350DRW is an 8' bed so I have a traditional hitch, but I've run into lots of folks with short beds using slider hitches who're very happy with them.
Good luck!