How much you make and how the commission structure works would depend on the dealer. The dealerships are individually owned, they aren't owned by Harley Davidson themselves.
Typically the winter is REALLY bad for selling motorcycles.
In my opinion, the market fell off for HD about 4 years ago. The market is saturated with low mileage used bikes, why would anyone buy a new one? The dealerships around my area used to get $3K-$5K over MSRP on the new bikes. Now they asking MSRP and I've known people who got bikes at under MSRP - that tells me they are starving. The dealers themselves are also offering incentives for customer loyalty, which is unheard from an HD dealer.
Again, I think this depends on the dealership, but I've noticed that the dealer I frequent has a lot of turnover with sales reps.