Don't forget to select 'with caravan (or trailer)' when using the route planning websites, as towing a caravan can cost half as much again.
Fuel cost differences can be much more than the 10 cents quoted by Ray. The cheapest diesel in the Pas de Calais area at the moment is 1.31 and the most expensive 1.52 - so 21 centre a litre with even bigger savings elsewhere! On a fill up that's quite a significant saving! There are similar savings on unleaded (SP95).
Fill up when you stop over for the night, which in our case, is well before we run out of fuel. We pitch up, set up, have dinner, and then find the nearest supermarket. We do our reseach beforehand using
http://www.prix-carburants.gouv.fr/ which gives you the latest updated information about fuel prices.