Welcome to FlyerTalk! Hopefully you find it useful here and stick around. Can you say what rates you've been finding, so we can try to beat that?
Also, what kind of SUV do you need? Just any SUV, or do you need a behemoth?
The first trial would be AutoSlash.com. For something that long, especially in a place with extremely high taxes/fees, you might be much better served getting a taxi to an off-airport rental location. Or get a 1-way rental from DEN to the off-site & then 13-day rental back to the airport, but originating off-site. Doesn't always help a ton, but it could.