The best time to rent a campervan in Bilbao is undoubtedly in summer. Of course, there will be tourists, but it will be much fewer than in cities like Barcelona or Valencia. In Bilbao, during the summer months, it is pleasantly warm to travel and enjoy the diverse mountain and beach landscapes of the Basque Country.
In autumn and winter, temperatures drop considerably, and there can be long periods of rain, but this does not mean that Northern Spain is not worth visiting. In winter, you can visit museums, stroll through the Christmas markets, enjoy a few pintxos in the bars and even, weather permitting, go hiking in the mountains.
The advantage of hiring a motorhome in Bilbao in the low season is that prices are lower and there is a larger selection of vehicles available.