I personally would not coat the underside of any vehicle with cosmoline. It's true that it attracts dust and dirt and that could be construed as a protective layer. But who wants the underside of their vehicle coated with grime?
The only way to remove the cosmoline is with kerosene or some other solvent. And it's a job and a half.
Why not just wash your vehicle after you're finished driving it in the nasty weather?
You might also consider that trying to work on the underside of the vehicle after it's coated in cosmoline is going to be a huge messy disaster. It's like molasses that can't be cleaned off of anything. Your tools are going to be full of sticky goo your hands and clothes are going to be covered in sticky goo and every other thing under the truck and in your garage is going to be covered with a disgusting mess.
Just clean the truck in the winter the same way you do in the summer.
1950 Chevrolet 3100 (Ol' Roy)
1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)