The Panasonic TM900 is easy to use for time lapse. Although it is a 2011 model, and being replaced, it is still easy to find. (At least Amazon has it.) Some are suggesting the newer model may not be as good!
I shot some storm days last winter that came out well with it's cousin the Panasonic SDT-750. I ran it from sun rise to sunset for three days without issue. The TM900 manual says it won't do 1080p in time lapse, but it does use a high quality 1080i setting. It also says that 2 minutes is the longest interval, so it falls short of your 15 minute goal. (That can be adjusted in editing software.)
It comes with an AC cord so you can run it off a wall socket, so battery life won't be an issue. It uses SD cards so getting the footage transferred to your computer would be easy. It also comes with decent software to collect, manage and stitch the daily clips together.
Where is your dealership and what do you sell? I'm currently in Lake Havasu, enjoying your AZ sunshine and a little of your wind.