I'll probably spend the winter down in southern Portugal/Spain where temperatures are around 15 degrees in average during the winter. I've also installed an old heating that works with Diesel and electricity. But yes, summer life is easy and I'm looking forward to experience my first winter in the van.
Inland Portugal can be freezing cold in the Winter, we got -2 C several days in a row last year (I know, -2 C is balmy elsewhere but try it without central heating). You should be okay if you stick to the coast as the sea breezes keep the temperatures in the 10-15 C most of the time.
If you've got a good sleeping bag, -2 C isn't that bad. I've spent the night with those temperatures in the high country of New Zealand. I know people that have spent the night in -15 C temperatures.
That may be OK for sleeping but it is very hard to work. I spent a couple of Winters coding without heating with indoor temperatures around 12 Celsius and it was very hard. I wore mittens, bubble hat, a blanket and even an electric foot warmer and still had to stop working and walk a bit every now and then to warm up.
BTW: I bought the van in February and already worked and slept in the van while it was freezing cold: https://ruby-on-wheels.github.io/blog/the-first-ride/ it was ok.