Hi Alex - my advice is Go For It, you will have a great time.
We went down via P&O Portsmouth/Bilbao last year, and came back overland. This year we went both ways overland. Both are enjoyable, it depends how much you like driving.
Turiscampo on the Algarve is an excellent site - lovely pool, nice setting, nice bar (beer and wine on draught at 1 Euro/glass) and restaurant, cheap wi-fi. On Camping Cheques it is a bargain - they do 6=7, 11=14 and 23-30, and you get a free bottle of wine. We stayed there three weeks this year.
Also been to Evora - nice site and pool. Nice town too, with lots of history. Only 10 euros in ACCI book - a bargain.
Last year we stayed at Vale Paraiso (also in Camping Cheque book. Nice site/pool etc but a bit dead when we went there in late April.
Also stayed on Municipal site in Monsanto Park Lisbon. Nice site, very convenient location - short bus or taxi ride to Lisbon.
I second Angela's comment about the motorway toll from Lisbon to the Algarve (A2). We paid 40 euros last year. This year took the National Road between Evora and the Algarve which was very good, and quiet.
Have also stayed en route in Spain at El Astral (Tordesillas), and Ruta de la Plata (Salamanca). Both OK - El Astral is the better site of the two, but Salamanca is a very nice (old) town, worth looking round. Both in Camping Cheques book. Both sites are a bit tight on access to pitches unless you have a motor mover
One site I would not recommend is Camping las Canadas (Camping Cheques no 562) near Bejar. The bar/restaurant is excellent, but the site is taken over by seasonal Spanish outfits - hardly any space for tourers. We were the only non Spanish there (nothing wrong in that, the Spanish are friendly, but do have large families, lots of children, and "enjoy" themselves late into the night).
Incidentally the road from Salamanca south to Merida is an excellent non toll motorway now,and a good way to reach the Algarve, via Badajoz (and Evora). Maps and sat nav are generally not up to date - the roads are being built very quickly, with EU money.
The Portuguese motorway from the border near Guarda is also very good, and toll free until you reach the Porto/Lisbon motorway A1.
The Algarve coastal motorway to Spain is also very good, and toll free.
Hope at least some of this is useful to you
Keith