I was concerned about this myself, because I'm going over for the first time in a couple of weeks, so when I was planning my journey earlier in the year I emailed all the French sites that I thought were potential stops and all of the municipal sites told me they did not accept twin axles, however there are plenty of others who do, and all of them had web sites and were more than helpful and I made all my reservations in advance by e mail. This is great because I've printed everything off just incase there's a problem when I arrive. Most have taken my reservaton on good will as its out of season, and some have just taken a deposit of around Euro10. I've even booked sites for the journey back in 2008!!!
Good Luck
Sian
PS I'm still trying to find out about the situation with gas and what's the best to do, if you've got any info on that let me know, thanks!