Potentially you could have a basement door that's leaking. However, usually the basement is colder than the rest of the trailer and there's a lot of bare steel in the structure that's wide open to condensation. We find that in the early spring and late fall, if we camp with electric heaters (to save propane when it's not too cold) the basement exposed steel is usually damp in the mornings. We always open the basement to help dry it out during the day and close it up before it starts getting cool in the evening. Possibly you've got a similar problem.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS