Hi james,
Many Spanish Sites have a storage area, but most people leave their outfit on their pitch for short periods of absence. Firstly to keep their place as Spain gets very busy in the new year and also because Spanish sites normally only become reasonably priced after a month, so it hardly pays to go on and off as you have to start all over again.
There are also plenty of storage facilities in the coastal area, probably shared with boats on trailers.
Portugal is quite different as in the winter good rates are available for short stays and most sites have a very cheap price for unoccupied units on site. This is normally shown on their tariff.
James