Are you going to stay only 10 days in Morocco?
I was this winter holidays there for 14 days and I found it to be a very short period of time. I slept in hotels/pensions. The average price for a single room is about 10-15 euros. I thought also about camping, but I needed some rest in the end of the day, because I was hassled nearly constantly. Merzouga (with a tarmac road from Errachidia) (where Erg Chebbi is) was fine, but very "touristy", not really what I thought to be the Sahara.
Just take the ferry to Ceuta, not Tanger, as it is "cheaper" (85 euros for a car and 25 euros for person ONE WAY) and the petrol there is really cheap (enter Ceuta with very little fuel only and full there!). Don't plan to sleep in Algeciras or Ceuta, as the only places where you can sleep are 3 or more star hotels (cheap hotels and pensions are whore houses -and very noisy-) and I didn't see any camping site.
Better don't eat at restaurants, take your own food as I had a 2week diarrhea after returning home and drink only bottled water.
Maybe Mhamid is a more sahara-like place than Merzouga, but there are no spectacular sand dunes (if you go there, be aware that there are childer trowing stones between Zagora and Mhamid) and I wont suggest you not to stop in Tamegroute (I didn't liked it at all - only begging children and nearly "compulsory" "guides".
If you use fuel, bear in mind that "sans plomb" means unleaded. Take a french dictionary if you don't know french and IMPORTANT: old clothes and whisky (bought duty-free in Ceuta) pour change (to change instead of money).
Have a nice trip.