Quote 1 (from a forum for caravanners):
"Last year, I used a cheap "value" kilo bag of table salt from Tesco/Asda/Aldi (or whoever has the best deal) in a pot dish. Cost about 30p.
Left it in the washing up bowl in the caravan sink. When I next checked it after about four weeks, the salt had turned to a solid block, but more importantly, the van interior was dry as a bone. Renewed with another 30p bag, and repeated monthly until we started using the van again".
Quote 2 (ibid):
"We use salt in washing up bowls and buckets placed around the cottage and caravan which are in France. It seems to work as there is quite a bit of water in them when we return. "
Source:
https://www.caravantalk.co.uk/community/topic/111328-cheap-d-i-y-moisture-absorber/
You'd get better results though by using calcium chloride, rather than sodium chloride, with something like this
https://www.wilko.com/en-uk/kilrock-damp-clear-moisture-trap/p/0135521
used with refills like these
https://www.wilko.com/en-uk/kilrock-500g-damp-clear-refill-sachets-2-pack/p/0410263
To save money, you could buy your calcium chloride in bulk
https://www.intralabs.co.uk/calcium-chloride/2kg-calcium-chloride.html
The same stuff must be available in Spain, although googling for 'cloruro de calcio' doesn't seem to find it easily.