Moisture on Bottom of doors and tailgate

Hi everyone, looked everywhere and can't find an answer to this question. Sorry if this is posted elsewhere but I couldn't find it. Here goes. My '14 Tacoma often shows wet areas on the bottom of my doors and tailgate as well. I seldom drive the truck in the winter (here in Canada) b/c of all the snow and some salt on the roads but I see the moisture daily. Just worried it will eventually rust out. I use Fluid Film twice a year and get to every part I can find and that includes the doors and tailgate. Anyone know why this happens and what I should do about it?
Thanks to everyone.

 

Ventilation my friend. If the truck is garaged then keep a fan running against it.

 

In 3 years it has never been outside overnight. Trying to keep it pristine that's why I wonder/worry about the issue. May try a fan since I have extra fans I could use. Thanks, I will give it a try.