I spent the winter of 2004/5 mainly in Portugal, but initially in Spain. I wouldn’t bother with Spain again – I’d go straight to Portugal, as you plan to do. I loved it. I only toured the southern, Algarve part, but it was absolutely delightful. Wonderful, friendly and ‘nice’ people, beautiful countryside, and lots of places to stop, away from habitation (and away from other motorhomes too!).
It depends what you are looking for, of course, but I recommend you do it, certainly the Algarve, where winter temperatures and sunshine are akin to our Spring/Summer temps!
Good luck.
Barry