Driving in Spain is a pleasure, except in the larger cities which it´s best to avoid taking a car into, certainly to the casco historico or old quarter which tend to have narrow and maze-like streets. Generally the roads are, compared to the UK fairly quiet and also have beautiful and far-reaching views.
Because Spain is a large and mountainous country there is always something to see, to stop for or to add an off-route exploration or two. The slow travel movement would endorse pottering around the Spanish countryside, exploring the white hill-side villages and camping in rural Spain under the stars.
As a rule the inland campsites don´t take bookings, but these days it is becoming more common. It´s highly unlikely that a campsite inland would be too full to accommodate you, but in holiday times they may be very busy.