Welcome, can I firstly say that you have made the right choice by going to LM and you almost certainly will be back.
Driving isn't a problem in my experience, I would say the roads are generally less crowded than UK roads, as Bella said, the peages/toll roads are recommended. Remember to pack reflective vests (1 each) and keep them to hand (back of the seats maybe) as this is a legal requirement in France, as are spare bulbs and warning triangle also headlamp deflectors if driving at night.
We always get our supplies from the huge Carrefour in Le Mans town, I'm sure your campsite neighbours will point you in the right direction.
I would suggest having a look at the guide published each year on
http://www.clubarnage.com/ This year's isn't there yet, but last year's will still be pretty relevant, it was invaluable to us on our first trip.