We used the A3 when we went down that way, we started from Urrugne in France and did a long journey as far as Honrubia and stopped on the truck stop at junction 166. Its not our favourite solution for night halts but we did it 3 times without problem, parked with the trucks and ate in a reasonable "greasy spoon" that the drivers use. It was toll free all the way from Burgos onwards. If you did decide to use truck stops ( there are not a lot of sites around there) dont go any further than the one sugested as the newer sections of the motorway dont have much parking.