A campervan in the Netherlands mostly sits still from November to March. In Portugal it is the other way round. On the coast near Porto daytime temperatures usually sit around fifteen degrees, further south they are higher still, and the solar panels keep producing right through the winter. How far a day of sun gets you depends on what you use, but it is a world away from a northern winter, and topping up at a hook-up is always an option along the way.
On top of that it is low season. The coastal towns are quiet, campsites have space, the mountain roads are empty and nothing needs booking weeks ahead. Portugal is also small enough to drive from the northern mountains to the plains of the Alentejo in a fortnight without spending whole days on the motorway.