Haha, well, like I said, I haven't really experienced Northern Greece at all, but Haldidiki looks fabulous! I probably wouldn't want to roll into Thessoloniki (or any large city in Greece) on a loaded touring bike, but I hear it's a nice place as well. I guess what it boils down to is whether you are interested in seeing a range of places in Greece, or would be happy just chilling out in a place like Haldidiki, which I've heard only good things about, and is big enough to offer a great deal of variety in terms of terrain and things to see and do and is small enough to navigate comfortably. Such a plan might be one option to consider. Remember, the lifestyle and pace in Greece is much more relaxed then most of us not living there may expect. If you have the need to make miles, then perhaps continue with the route through the mountains of Greece down into the Gulf of Corinth, you'll still experience some nice views, and come through some amazing towns (I have been across the Gulf on the northern side, and there are some very picturesque towns dotted along the way, and you're guaranteed some fantastic sunsets.)

As far as Peloponnesos is concerned: In my experience, I have seen camping facilities, and small towns with places to stay along the way on the old highway that runs parallel to the coast on the Peloponnese side. I have seen people ride this rode, (I'm not sure if they were locals or tourists) but again, please be aware that truck traffic could be a major issue on this road as it's very narrow, even for a national road. If you find through your research that the roads will be better north of Peloponnesos, then I would stick with that plan.

BTW-- the leg of third leg of Haldidiki, Mt. Athos, is off limits to women, so you might want to take that into consideration.