Op, maybe tell us whereabouts in Brittany you are going and there will be lots of advice on places to go, shops etc.
We have a touring caravan, after a string of hols in Cornwall when kids were early primary, we took the plunge and went to France. A colleague of mine had been with his caravan many times and I wanted to try it. DH only agreed I think to get it out of my system.
Well that was about 11 years ago, and we have been back most years, apart from a couple of long haul hols and a year we needed to stay in UK. We would have gone this year but have had a family tragedy and a wait for the funeral.
Our first trip was to Sarzeau in south Brittany and we have been back there, plus also been to the Loire, Dordogne, Royan, Paris and Vendee areas. As others have said, the roads are much quieter than in UK, even motorways. The only problem we have seen is that some motorway service stations are heaving on summer Saturdays, but given that you are doing an overnight ferry I am guessing you wont have far to drive at other end. We get around that problem by travelling midweek, as having a tourer we are more flexible. I have towed in France, and driven without the caravan on, though tend to let DH do the manoeuvring onto tunnel or ferry.
We have done most ferry routes, including a short overnight from Newhaven to Dieppe and a long day from Roscoff to Plymouth, plus the tunnel.
Your site will most likely have a shop and/or a delivery of fresh bread (baguettes, pain au chocolate, croissant etc) in the mornings. Supermarkets are brilliant, the patisserie is to die for, but also lots of lovely fish, meat, cheeses, salad etc, not to mention wine! Moules Frites (Mussels and chips!) is a staple in Brittany restaurants, along with other seafood, my two have loved MF since our first trip, when they were 7 and 9.
Most towns have a market once a week, selling all types of food plus maybe clothes and local products. They will normally have food stalls too, selling hot meals plus crepes and other snacky produce.
I am getting quite jealous and really wish we were heading there ourselves, our neighbours left yesterday to go. But it doesn't work for us this year.