The furthest distance in Morocco to travel is just under 200km, but although diesel is always available petrol may not be there, in this case the furthest stretch I can think of is 300km. Fuel is available at the turn off to Dakhla and at two other stations on the way to Nouadhibou.
Nouadhibou to Nouakchott, there may be petrol available in Nouamghar (330km sand from NDB) but it is not guaranteed and will be expensive. It is used by the local fishermen to power their boats.
Going east you may have problems, fuel may be available in Choum but I wouldnt bet on it and certainly not rely on their being petrol. The vst majority of cars in Mauritania are diesel powered so keep this in mind.
If you are going to drive down I think going solo on a bike is taking a big risk anyway, if you were to go with other cars or a 4X4 then they would usually have no problem to take extra fuel for you.
Petrol is available in Atar and towns on the Route de l'Espoir and sporadically available in Chinguetti and Tidjika, but again I wouldnt bet on it.