Oldtimr

It is not at all unusual for a cabin to have a musty smell it is has been closed up and not used for while and no fresh air has circulated in the cabin. Unless you see mold you don't know if you have a mold problem. Dampness in an unheated building gets into mattresses, furniture etc causing a musty smell. It may be as simple a fix as opening all the windows for a day or so and taking the stuffed furniture and mattresses outside to air out, or replace them, if you don't replace them spray them with Febreze, you can also get Febreze to spray the air with to freshen it. Don't jump to the conclusion you have a mold problem, what you experienced is not unusual. If you are worried about mold you can have someone come out and test the air inside the cabin for mold spores. Some people if they can afford it leave a w heat on in the cabin when they are absent. You could sit a few containers of desiccant around the cabin when you leave each time. You could also talk to someone in the building trades about how you could add some kind of ventilation to the cabin to help with the problem. I am sure there are others here who have dealt with the same thing you are.