With the caravan we also got an original Osokool evaporation chiller.
From what I have been told you place a cupfull of water daily onto the circular indentation on the top and this keeps the unit cool. The inside is lined with metal (see picture) and we have the original packaging and repair kit too.
Cooling units powered by natural evaporation, clean & eco-friendly.First built in 1962 these are environmentally friendly coolboxes,which work on the evaporative principle,the transfer of latent heat by absorption. No gas or electricity is required just a cup full of water added daily to keep the liner moist. The water is then carried around the inner cabinet & as it evaporates the interior is cooled.It is not a refrigerator but it will continue to work long after an icepack has melted in a standard coolbox.
Fitted by motor caravan manufacturers as original equipment in the 60’