If that's Wern Farm campsite at T'yn y groes it's fairly local to us, would take junction 19, off the A55, then A470 to Tal-yCafn where you turn right over the bridge to T'yn y groes.
You could carry on and come off at Conwy on A55 but you would be going through the castle gate which is very narrow so junction19 a better route I think.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.