First of all, there are very few places you'll get into trouble if you sleep in your car or a van, basically vehicles, where it's not obvious, that you're camping in them.
With a campervan/motorhome however you'd get into trouble immidiatly in places where they don't like you setting up at the side of the road.
As said by others: if you don't rub it into peoples faces, you'll be alright. Especially in areas like truck stops etc.
I drive home to Denmark once or twice every year from Ireland via UK/NL/Germany and do exactly the same. It's just more comfortable in the back of a van with a madress
/M
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