We are looking at going to Moroocco in the later half of sept. Will it be too hot?

It won't be too hot, but it will be hot. It will cool off somewhat at night, but it was quite hot pretty much everywhere when we went that time last year (late Sept, early Oct).

We did a loop of Rabat, Fes, a 4 day tour )thru the desert, Todra gorges, etc), Marrakech and back to Rabat in 10 days. Took trains between Rabat and Fes and Marrakech and Rabat.

While it would always be nice to have more time, we didn't feel rushed. So, two weeks should be fine.

Can't comment on December as we went in early October (it was quite hot then).

I went in Novemebr and chose to stay in a hotel rather than a camp for the desert. I was pleased with the decision. I would give minimal time to Casa.

Thanks for the input, we really want to experience the dunes night, but also do not want to freeze.
Thanks again

I don't think you'll freeze in any event. The tents we had were nice and adequate bedding was provided.

It was hot when we went to bed (early October), so we all dragged our sleeping pads outside and slept under the stars.