Santander to Tarragona. Best route to tow in January?

[Stevemr]

Towing a caravan from Santander to Tarragona end of January.
Don't know that part of Spain. I think it’s quite mountainous though.
Which is the best route? And roughly how much are the toll roads?

[GreatGranny]

The north is pretty hilly but it's all dual carriageway according to Google maps (route takes you along the coast to Bilbao then south).

They look like steady climbs, nothing extreme.

Further south you go the flatter it gets.

There are a few websites for calculating tolls.
Just a car it says 35 euros, add maybe 50% for caravan?