Hmmm...this is just my opinion (as one of apparently few women who loves shopping and negotiating for vehicles), but you can't really ask you have to tell. If you ask, they will almost certainly say no.
Not sure what taxes/fees are like in your area, but if you're happy at $14K I'd either offer $14K OTD or $15K OTD if you feel that is more "fair". Personally, I'd be going $14K OTD.
Obviously, how hard you push depends on how much you want the vehicle and how much ability you have to walk away. For our RV, which was a new leftover 2007 that we purchased in 2010, I only got them down 11% but that was OTD which saved additional cost. While we had a few to negotiate against, they are rare units and we definitely liked this one the best, so we didn't play ridiculous hardball. We would have been okay with the others, but we also would have been willing to pay the asking price. ;)
Also, you may consider making the deal contingent on an outside mechanic/RV expert inspection.