We did this last year! Good timing for once. All self catering though.
Useful things we took:
Small plug in car fridge from Halfords that can be strapped down using the middle rear seatbelt. Saved having to buy new butter, milk etc every time we moved on to new accommodation. Also useful for packed lunches.
Kitchen stuff packed in a plastic box with a lid. Along with the usual washing up liquid etc, take a small knife and a big knife with removable covers and a chopping board as the ones you get given are always rubbish. Small sandwich bags also v useful for everything. Corkscrew. Matches.
Get a usb charger block with 5+ outlets for all the gadgets. Pop it in a little box with all the relevant leads plus one euro adaptor. Voila.
Hammam towels.
Everyone gets a transparent Ikea laundry bag for their clothes instead of a solid suitcase - easier to squash down. Blue Ikea bag for laundry.
Picnic rug always packed at the top!
You’ll need to take various bits of paper (V5C, green card insurance thing, travel insurance, European breakdown cover) - check this depending on where you’re going and what happens after Brexit. Then stick it all in an envelope thing and keep in the glovebox - obv take it out and with you if you’re parking anywhere dodgy!
You’ll also need stuff like headlamp converters, hi-vis vests, first aid kit etc - look this up for each country.