I do have some experience of the coast east of Roscoff, but more by yacht than motorhome. As Witzend says, the Côte de Granit Rose is definitely worth exploring; I particularly liked Ploumanac'h, but believe that there were (and probably still are) some restrictions on motorhome parking. Tréguier and the river Trieux (Pontrieux, Lézardrieux...) are also worth exploring. However (as Witzend also said), the weather may not be optimal at the time you are likely to be there. If the weather is better down there, you might consider south Brittany, (Concarneau, the many small coastal aires in the commune of Névez, and the area around the Golfe du Morbihan, including the alignements at Carnac). Wild Camping has some excellent POIs in Brittany, particularly coastal and adjacent to inland waterways.