It is a little hard to give a per day rate but the following observations might help.
Food? We cook mostly and eat at restaurants occasionally whether we are at home or in France. So our food bill is slightly less in France (compared to Rep of Irl)
Drinks? The cost per bottle of wine in France is lower but we do tend to drink a bit more in France than we do at home - something to do with getting into the culture.
Diesel? When we were there earlier this year I recall diesel could be bought at supermarket pumps for as low as €1.29 whilst in Irl we were paying over €1.50 so €0.20 to €0.25 per litre saving. France is a big country and you can rack up a lot of miles
Tolls? Very expensive and there are lots of them - This site (http://www.viamichelin.co.uk/ ) can act as a route planner for you but will also show toll costs (it is approx because there isn't an option for a motorhome for type of vehicle)
Obviously you can control cost of tolls and diesel by driving more or less and during this kind of holiday don't be tempted to spend your holiday driving.
Campsites are a biggie - but even they depend on the time of year you go there. Once upon a time we used go in high summer when our children were small and we could only get our time off during peak holiday time. Now we never go in July or August.
In 2014 the ACSI sites ranged from €12 to €16 per night and that usually included electricity. Even non ACSI sites in the non-peak season offered roughly similar rates. Sites close to popular destinations tended to be more and you can pay €20 to €30 per night. A good alternative are the aires and there are loads of them ranging from free to a few euro per night. You can also wild camp - just use common sense. Over the course of the holiday a mix of camping sites and aires and wild camping will keep the costs down. Some people as a matter of principle never go to camp sites! If you do long stays on one campsite ask if they have deals. Some owners will give a good discount less than ACSI rates if you stay with them for two or three weeks - or longer
Aside from that you may spend on paying into various tourist attractions.
Phone calls - only family and we text our family to call us.
The only other item we spend a little money on is we rent a small car from time to time if there is an area we would like to explore more fully but don't want the hassle of driving the MH around.
Lastly - as with all foreign travel - it is advisable to have credit card which might get you out of an emergency eg. if you had a breakdown
Trust me. You will love France. And you will also love Spain and Italy too.