It is hard to tell as there are so many different parameters. I can only speak for the Caravan and Motorhome club but I am guessing that the Camping and Caravanning club is the same, but the CMC have discounted fares on certain sailings at certain times of the year and on certain routes. There isn’t a blanket discount as far as I know. So for example, the last time we used Brittany Ferries, we discovered that the CMC had a “caravan’s go free” deal where you only paid for the car and passengers but it was only on the afternoon ferry on a Monday and Tuesday which arrived in France at about 10.30pm. If you were happy with that setup then it was a good discount but if you wanted to arrive in France at a more sensible hour to find a site to stay in on the first night or you wanted to travel on different days, then it wouldn’t work for you.
Edit: I should add that it’s been a few years since we were last in France and therefore my reply is based on our last experience which was the year before Covid.