My husband and I are retired and living in Europe in a small camper van. We will have about 6 weeks to see Spain and Portugal and will be driving in from southwest France. We speak no Spanish. What is the best way to combine the two countries without excess driving since diesel is so expensive. We are more interested in culture and history than the beach. Thanks.
As far as I can tell there is only one way of combining Spain and Portugal and that is to drive all the way across Spain, a matter of around 1300 kms/800 miles. Other than that, the choice is up to you. You can enter Portugal via the south, the centre or the north, depending on where in both countries you want to visit.

It'll be a long drive whether you like it or not, as Tony already explained. The only way to keep your mileage low is to stick to the norther part of both countries, but since some of (especially) Spain's most interesting sights are in the south, that wouldn't make much sense either.
Consider making a loop via northern Spain and Portugal south towards Lisbon and then onto Andalucia before heading back via Madrid towards Barcelona. Something like that, maybe?

Yeah it the first two post are right, it's going to be a lot of driving no matte what you decide to do, but remember you're on vacation. If you don't see all that there is to offer the trip is kind of wasted. You should look into different modes of public transportation that may take you to the some nice destinations for a fracture of the cost. Maybe catch a bus or train to certain tourist areas, that way you eliminate the driving and worrying about gas.