Fascinating thread! Commonsense is the rule IMHO. Our 2 year old Goldie loves the van and has been on every UK holiday with us since we got it. She travels really well and we have the D anchor in the seat rail and a shock absorber leash connected to her harness. In my view there is a world of difference between leaving her in the van on a cool campsite in, say, Snowdonia, with zero chance of overheating, compared to the south of France in 30c + temperatures. Near to home, unless it is extremely hot, I confess I sometimes leave her in the van for a few minutes while I pop into a shop - why wouldn’t you? Sometimes the wife and kids are left in there too and they don't seem to complain if I come back with the right sort of treats
Having said all that, we recently planned our summer holiday to France in August and reluctantly decided that it was going to be too difficult all round to take her all the way south. Too hot for comfort really and added to this I read that although the French love their dogs, you can’t take them on the beach in most places! This would make life miserable for her and our 13 and 6 year old kids! So we were looking to change the itinerary and head inland to lakes and mountains (maybe Auvergne region) where it would be cooler and she could get wet. But in the end, parents have come to the rescue and will house/dogsit while we are away so she will be spoilt rotten. Although we will miss her, its the best compromise for everyone to be safe and happy. Plus it will save all the hassle and expense of the extra vet bills for jabs and checks. Mind you they can get into enough trouble all by themselves - I just took her to the vet today to see about a limp and its looking like a damaged cruciate and a massive bill if surgery is required
Only you know what’s best for you and your pup. Have a great trip - and if he doesn’t go on this one, there will surely be plenty more for him to enjoy.