Negotiating 101: you need leverage and a price threshold you are willing to accept.
Find more than one dealer you are willing to work with and start conversations with them. Engage them all and begin low. Start at 70-75% of MSRP and work your way up. If you are willing, consider some other brands/models that might suit your needs and seriously look at them. Know similar models and be able to communicate to the dealer that you have options and are looking for the best deal (as opposed to a specific unit that you "have-to-have"). You don't have to necessarily be willing to buy the alternative, but you need the dealer to believe you have other options. Also, what is your timing?...if you are willing to be patient, it will help you in negotiation.
Make your initial offer and wait for a counter-offer. Don't negotiate against yourself, but make it clear that you will be a reasonable buyer with serious intentions. Be patient and be willing to walk away if you aren't getting the response you want that day. Come back in a few days and see if they are willing to continue the conversation.
I can't say that I know what you will be able to get in this market. New dealer sales don't seem to have the deep discounts they had in late 2008/early 2009 when they were trying to get rid of inventory. Plus you are buying a high value, high reputation brand and the dealers won't have the same motivation to move inventory. I was able to buy new in October 2008 at around 65% retail, but that was a different market and a different brand. I doubt you will be able to get nearly that discount on a Tiffin.
Lastly, if the above fails and your diligent negotiations aren't progressing, you have a choice. Find a different brand/unit, or if you have found the unit you and your DW have to have...just do it. Life is too short to compromise your dreams for a few pennies here and there.
Others may have differing opinions, but that's my $0.02.