Terry - have a look at the
Camping Cheques website - at the moment they're doing an offer with 'Caravans go free' with Brittany Ferries on selected routes (Plymouth to Roscoff is one of them). It may not save a fortune, but perhaps two or three nights' campsite fees. If you don't use the Camping Cheques you can always put them up for 'grabs' on here - someone is always interested.
Also, unless your holidays are fixed, by work commitments, etc., then it may make sense to look at earlier or later dates. During the peak holiday weeks (and you're coming up to them first two weeks in July) ferries, sites and roads are more crowded. Two weeks earlier and everything will be much quieter (and perhaps cheaper) - and you may be able to take advantage of Camping Cheques or an
ACSI card. These can bring down the cost of sites significantly (with sites costing between 11 and 15 euros, inclusive of pitch, car and caravan, two people, awning and electricity).
I haven't done the long crossings, but good luck! You'll get plenty of good advice from the contributors on here.