If you can feel an actual air infiltration, then the seals are probably leaking. If debris gets into the tracks, it can sometimes result in the window panes not full seating and causing an air leakage. Cleaning out the tracks can sometimes be a cure. There is a seal between the sliding glass pieces. If this is damaged, a thin, self adhesive weather strip foam piece can be added. Your trailer isn't older. Some that are have had their window seals dried out and become stiffened by UV sunlight. As they loose their flexibility and size, gaps will occur where there once was sealing.

You can temporarily seal the window gaps with a household silicone during the colder weather, and peel it off during the warmer summer months. If there are no air gaps, and the cold is caused by a single pane glass installation, there are a couple of things you can do. One is to install an inside "storm" window made of acrylic or polycarbonate materials. They are easily sized and shaped to match your windows and fastened to the inside window trim. It's an easy DIY project. Another air barrier is the addition of a heavy curtain material.

All of these are temporary fixes to mask the window sealing difficulties. Most window seals can be replaced, with parts from the trailer's OEM.