We tow a 5 metre SIB to the South of France or Northern Spain each year. I'd need to check with the wife but I'm pretty sure there's no extra to pay for a boat on a trailer at the tolls. A quick search on the internet reckons it will cost approx 100 Euros for Calais to Frejus which sounds about right.
Routes: preparing your routes on french motorways - ASFA
I'd imagine that it would be much less expensive to take your own RIB rather than renting one whilst you're there.
The place that we go to in Northern Spain has a beach that we just leave the boat on each night. There's a metal post on the beach and we just pull the boat up the beach and chain it to the post. I'm not sure if that would be feasible with the weight of a 6.2 metre RIB though.
We've also been to a site near Frejus in the South of France which has a river that leads directly to the sea. The site has a limited number of mooring berths but as long as you get one it's perfect.