Id say that nowdays , there are all petro stations all the way from Morocco to SouthAfrica never more than 100 miles appart . Take jerries if you with , but they are as pointless as a GPS nowdays
Unless you plan to do something really radical (freepisting in Niger , or crossing the big-sand-sea , as long as you follow roads , there will be petrol stations.
Regarding buying subsidized fuel in Western Sahara.. , well ,Its indeed cheaper (50% than in MAuri or Morocco ) but does it really matter ? I mean the cost of a jerry (15 Euros) is more that you will ever ever save by buying petrol in the cheapest country. Do you really want to carry 100 kg extra in your roof-rack just to save 40 Euros ? (excluding the cost of the jerries themselvers)
Also , don't buy those Moroccan plastic yerries (40 DH , 4EUR) They are intended for waTER , or for camel-shit (really ) , but never put diesel on them . The caps doesnt close at all ,they leak everywhere , and one of these ruined one of my trips when It pop and poured 30 litres of diesel over my , my car , my luggage , my clothes, food , etc.. The car smelled like a fishing boat for at least 1 month .
Same about waTER. I ussually carry heaps of bottled water (ussually from Spain ) just 'cos they are hell cheaper than in Africa. But honestly , I think that you can get water (even bottled water) everywhere in your route. Carry a lot if you wish , just to feel safer , but there's no need at all.