We have two fantastic fans and two large skylights in our camper. All are double pane. During our winter ski trips condensation will still form on the inside and you can feel the colder air.
We opted for a solution using Reflectix which we seal with 2" blue 14 day removable masking tape.
We tape the edges of the reflectix with wide packing tape and use double sided tape to attach the layers together as show below.

1st layer fits inside; 2nd layer covers the frame, 3rd layer extends reflectix to 2-3" beyond the edge of the flange or frame.
Below you can see the reflectix taped up to the vent. We do the same for the skylights.

The multiple layers and the tape prevent water vapor from getting to a cold surface and condensing. We only install these at night when it is colder and take them down during the day. Easy to store the reflectix under the bed. We also do the same to the double pane windows in the camper as well, but don't take them off each day. We made a storm window for our dinnette table window (effectively triple pane) and then also use the shrink film as well (four pane) so it is light and bright inside during the day.