I know quite a few good sites in North Wales but I'm afraid that I don't ride a bike when I go camping so I couldn't comment on the roads. Anglesey was very nice, and I once stayed at a campsite just outside Bangor (Treborth Hall) which was in easy travel distance of Bangor, Caernarvon and parts of Anglesey even on foot.
the site in Llangammarch Wells near Builth wells )Riverside) is on a marked cycle route. I live in the village but can't for the life of me remember what route number it is!!!!!
As mentioned Graig wen is good just off the Mawddach trail but we preffer Hafod Dywyll farm campsite . good access to both trail and quiot lanes round the foot of Cadair Idris and Dolgellau. depends on what type of site your looking for ? but the views are stunning . this is the veiw we had every morning