freespirit , sounds like I guessed right about you having the regulator on the bottle system !
are you taking a laptop computer ? if you are , go here
CAMPINGCAR-INFOS
as you can see , you can look for free any time , but for a few euro's you can download this on to your laptop or a usb stick [ or buy on a usb stick ready loaded ]
this guy has done a fantastic job and his data base is in a class of it's own ...you will find it invaluable
here is the energaz site for when you come back to france
énergaz les nouvelles bouteilles butane & propane
as you can see there are points of sale everywhere ...although it says the price for the bottle is €5 I have always been charged €1 for some reason
with regard to a route across france in december , the weather is , and always has been , unpredictable ; in southern france we have had temperatures as high as 25C in november ...but as low as -17C !!!! so the important thing is to choose a route where you can get through if the weather turns against you [ hence leaving the uk with propane rather than butane ]
what tends to happen in france is that the valley routes get blocked if there is snow , but the west coast and the massif central route normally keep open ; personally I don't like going down the west coast as when you cut across to get to the med the route is not the best [ I don't pay for motorways ] ; in any case it is [ I think ] a long way round and if you are coming north from portugal in the spring you will be using that route anyway
my preference is [ presuming you are coming from calais/boulougne/dunkirk /le havre ] is rouen [ motorway from abbeville to rouen is free ], then chartres , orleans , bourges/ clermont ferrand and south to the med on the A75 [ which is free except for the millau viaduct which is one of the great sights of the world ...google it !] ; I've been over it once , but usually go through the town and stop at the aire ; the A75 is fairly new but has never been closed as they have more machinery than you can shake a stick at [ funded by the EU !]
anyway ...enjoy