Camping Municipal de Perharidy
Route Santec, Roscoff, Brittany
This is very close, west of the port, and lies behind the sand-dunes of a silver sand beach. Google earth will pin-point it for you. We had a free night there with the ferry booking that docked late. Not sure if that may still apply, but we liked it here and there were fresh baguettes in the morning!
Don't know whether you have used Roscoff before but the dock is in open land and you are very quickly onto the road network with near to zero traffic. On the return journey you can see the ferry for several miles across the fields! Pity the poor unsuspecting travellers that are coming the other way into the bliss of Plymouth traffic instead!