Driving to Italy Luxembourg & Switzerland offered the cheapest unleaded prices and were at the right distances apart. As in UK expect prices to be higher on motorways than off route stops
On driving to Spain there was no choice but to fill up in France and the only cheap deal I got was on the return leg in Andorra. Had trouble about 8pm at night getting fuel around Boulogne. Finaly just made it to the hypermarket on fresh air.
Stop at regular intervals even if you don’t feel like one, being the only driver I allowed about 100 miles regardless of the length of rest time or for what reason. There are two types of motorway stops, normal Services with fuel and shops or ‘P’ parking just to rest up often with toilet facilities.
Drive in very casual attire even if you feel slightly chilled it helps keep you awake and alert
I take sandwiches and drink in a cool bag and more in a cool box kept closed to keep them fresh, and it works a treat if taking 2 days for a journey and to save money.