First question is whether you are starting from the actual factory MSRP of some dealer "list price". The factory MSRP is supposed to cover dealer prep as a necessary part of the sale so only delivery (transportation) charges are added to that. Some dealers add additional services (like exterior wax and interior "protection" or charge for delivery or prep, on top of whatever they claim is the list price. It's a jungle, so be cautious.
Buyers are claiming typical discounts in the 22-38% range, but are vague about how that is calculated. It's easy to get a steep discount if the dealer then adds back fees for services or if the price was inflated to begin with. It's more important to get a bottom line (aka "out the door" price that includes all the delivery/prep stuff and leaves the dealer enough margin to do a quality job of it, plus time to show you everything in the RV and how to use it.
You can ask for things to be "thrown in", but at the end of the day the cost of that has to come out of the discount. Sometimes a dealer can throw in something that costs him less than you would pay if you bought at retail, but with online discounting that is not often not the case. If you can buy & install something yourself, that is often a better deal. But if you are going to ask the dealer to install anyway, you could negotiate the price of that service as part of the deal.
Dealers love to sell add-ons like extended service plans, tire insurance, surface protection (wax), interior protection (Scotch-guard),, etc., usually at high mark-ups. Most are either not needed or much cheaper elsewhere.
The Grand Design doesn't come with slide toppers? Seems odd for an upscale brand, but it is what it is.