Emmerson, If you can find the right deal on P&O Dover Calais you have no worries about being late for a ferry as they cross every hour. Cheaper crossing during night if you book it right.
Auchan , E LeClerc and other main French Hypermakets sell LPG, I looked at the price at Auchan LeMans last year and nearly fell over it was so cheap. Hypermakets have pay booths at exit generally and not a lot of room if any for a caravan. On my travels I've seen brits unhitch vans to get LPG and diesel at Hypermarkets. And remember fuel stations close in the evening and at weekends and UK chip and pin cards still work in few 24/7 pay pumps!!!!!!!!
You can get cash at Hole in wall machines at banks and hypermarkets same as here, And instructions do come in English if you didn't know!
We mainly fuel up and eat on route at hypermarkets rather than motorway stops with the van and when working.
www.viamichelin.com will give you routes and you have the option to expand the route to show town layouts, you can choose road types and towing caravan options. And you can get road work and hold up details before you leave. Sign up as a user as it is free and more options are open to you. RAC and AA route finders are OK as well, but I find Michelin web routes good.
Bon Route!