Again only with a motorbike and not a Campervan, but I've been over a few crossings.
From Wales, and if heading SE across France, I found its nicer to head to Newhaven-Dieppe. I can go across from bath & Salisbury across country to Newhaven and enjoy the journey far more than the M4-M25 trudge to Dover. (Ive never done the Chunnel btw - I like a Ferry Crossing!).
For Brittany and central France routes then I found Portsmouth/Poole to Cherbourg (think I prefer Poole btw) to be a good compromise between costs and point of arrival, and better value than St Malo - its maybe 3 or so hours on the road to make up for the fact that St Malo drops you further South (plus you can take in the likes of Mont St Michel en route if heading towards Brittany). But... St Malo is a really nice place to arrive at - most Ferry Port towns aren't much to write home about, but St Malo is terrific and worth a visit even if you didn't land there.
Last time I crossed to Cherbourg it was to ride down and follow the Loire for a bit, then head into the volcanic central lumpy bits, then South to Millau, along the Tarn Gorge east to the Ardeche, through the Alpine roads across the Col'D'Iseran to Aosta in Italy, then over the St Bernard Pass over Mont Blanc back into France, then we worked our way up to Calais (simply as my friend on his bike wanted to visit his son in Kent on the way home).
I can't say I'm enamoured in the least with the journey to Dover, or the miles out of Calais, and neither port is exactly inspiring. So for me, going in and out from one of the South-Western Ports made the first days and last travel to and from the embarkation port part of the holiday and less of a chore than the grind to Dover, plus you tend to land straight into some more pleasant surroundings on the other side.