The following parts are UV mapped backwards, so you'll have to flip your design for it to show up properly:

Painted Sideskirt (28 feet) - left and right
Painted Sideskirt (53 feet) - left and right
Painted Sideskirt with Hole (28 feet) - left and right
Painted Sideskirt with Hole (53 feet) - left and right
Aero Sideskirt - left

The following parts are mapped correctly, and DON'T need to be flipped:

Painted Sideskirt (45 feet, 48 feet & 53 feet 4 axles) - left and right
Painted Sideskirt with Hole (45 feet, 48 feet & 53 feet 4 axles) - left and right
Aero Sideskirt - right

Note that "Painted Sideskirt with Hole" refers to the extra sideskirt available on the left side for Reefer trailers only. It has a hole to access the fuel tank, but is just called "Painted Sideskirt" in game.