Have you tried a search using Google, I came up with three addresses but further info was all in Portuguese. You could also try the Orbitur chain of sites, they have a least 4 sites in northern Portugali. They may do storage?
Try the tourist office in Porto as well and ask if they know anywhere in the city and surroundings - but check with your motor insurance company about insurance cover if you leave your campervan overseas.
We stayed at Orbitur Angeiras, Lavra in May & there were plenty of 'long stay' outfits there.... some had been there so long that they were rusting away or being taken over by the surrounding vegetation.....
Skingers1