March/April this year a lot of campings were shut, particularly in the Northern half of France (in fact, France was pretty much shut). Going south I found a few open (around Bordeaux/Arcachon area) but it is worth considering what you will do when you can't find one (Wild camp? Hotel?)
North coast of Spain is amazing - food, people, scenery. It is pretty tough going though - lots of up and downs (think Dorset coast). I avoided big towns/cities mainly, but riding into Bilbao was pretty cool, past the Gugenheim and into city centre. I'd recommend a stop there.
North Portugal I found a bit of a different kettle of fish to Spain - roads/traffic not as nice, towns/cities a bit dirtier and not so welcoming feeling. But then again it of course all depends on what happens with you - I could go the same route next year and really love it.
Porto was ok, easy to navigate into/out of, but the beach/seaside town of Espinho about 4 miles down the coast is a much more relaxing stop.
The hills between Portugal and Spain (I went back across central Spain) are big and tiring (especially in heat) but beautiful - watch out for the dogs!