Carcoon Or Similar

[Studley Ramrod]

Sounds like problem you might be having is water ingress from the roof causing the walls to get wet and stay damp.  If so then sealing up the air  gaps around the doors and roof etc won't necessarily stop the damp atmosphere and could make it worse from a lack of ventilation.

 

I would prolly build a damproof shelter inside the building and then, if you have a leccy supply, install a dehumidifier that has a built in humidity meter and that also has a pipe fitting to allow the unit to drain off externally rather than drain off into a holding tank.