Hi Diane
The the motorways and the non-motorway trunk roads are good and the trafic is light compared to the UK. If you are crossing the border at Guarda the motorway heading South West from there is good and free of tolls. The South Coast A22 likewise. The North / South Motorway, Porto / Lisbon / Algarve is very good, but also very expensive in tolls (they double the toll if you are towing a caravan). Minor roads, other than major trunk routes are very risky, ie. can be narrow, bumpy and even cobbled - best to avoid these.
Another good way to reach the Algarve is to follow the new (free) motorway South from Salamanca towards Seville, but pick up the Madrid to Lisbon motorway half way down and cross the border on this. It has a toll in Portugal but you can turn off at Evora (good campsite) and take the non motorway route South.
Hope this helps
Keith