Black Stripes on Ceiling

[newlandsj]

Black stripes, almost looks like shadows, have appeared on the ceiling. They look to be exactly under each cross bean of the roof. There is no water leakage anywhere inside the RV. Wiping with a mild detergent doesn't do anything. Any thoughts as to what is causing this and how to get rid of these dirty looking marks? Also we do run a dehumidifyer all the time inside the RV.