My boyfriend I are planning a trip to Tunisia from May 30-June 15 of this year (08) and had a few questions that we are hoping some of you may be able to help us out with.

Our current plan is to stay with a friend in Tunis for 2 days, then rent a car and drive to Tabarka (1 night), Le Kef (1 night), Karouan (1 night), Sbetla (lunch), Touzeur (1 night), Douz (1 night), Ksar Ghilane (1 night), Matamata (1 night) and Gabes, stopping at a few other towns along the way and spending the night in the cities that I have marked. From Gabes, we were going to take a train up the East Coast, stopping at Sfax (2 night), El Jem (lunch), Sousse (1 night), Hammamet (1 night) and Sidi Bou Said (1) eventually reaching Tunis where we would catch our flight back to the US.
Our first question is, does this plan seem feasible over the course of 15 days? While we want to see as much of the country as possible, we want to set our own pace and still have it be enjoyable. Is this too aggressive? We tried to allow ourselves a bit of padding time in case we wanted to spend an extra night in the Sahara or something along those lines. Our second question is are we picking the right cities to spend the night in? For example, is it worth it to stay in Douz and then Ksar Ghilane, or should we just spend the afternoon in Douz, look around and move on? Is driving problematic once we reach the south? I know the NW is supposed to be quite drivable, but we will not have a 4x4 and while I know there are roads connecting the cities I would prefer to not get stranded in the Sahara! We are trying to find a GPS, we may bring our own and buy a map of Tunisia for it but we would prefer to rent one. Does anyone have any experience with this? Is planning on driving from Karouan to Touzeur in one day possible? Lastly, does anyone have any ideas or pointers for us? We have traveled in some off the beaten path areas but never fully by our own planning. Any tips or anything like that would be hugely appreciated.
Thanks in advance for any help you can give us!