Mid week, non péage can be empty but there's 3 issues:
They can be single carriageway so you have to be a keen overtaker
They are a popular gendarme hunting ground, if you get caught speeding it could be very expensive
If you're going across half of France you can save autoroute money at the cost of a few hours, but if you're going across all of France it's probably easier to pay the péage fees for the simplicity.
Yes and no.
Mid week, non péage can be empty but there's 3 issues: