New Year before last, we went to Belgium and France between Boxing Day and New Year's Day. Did not use campsites but plenty of Aires available. We stopped in Bruges (now expensive aire) then Ghent, Antwerp and Tournai free of charge. Then back to St Omer for New Year's Eve in an outstanding restaurant. No aire there so we parked in a town centre car park. No problems and no-one bothered us (although we did have a rousing New Year welcome from some local youths on our way back from the restaurant). The only thing to beware of is that not all the 'Bornes de Service' are operational in mid-winter so you may have to search around for a place to dump and replenish. Again this is not a problem if you use locations close to towns rather than villages.
Philip
p.s. we have 2 x 110 Ah batteries and seldom use hook-up, but realise this may be a bigger issue for you. We can last 3 or 4 nights without re-charging even in depths of winter (no solar or gennie).