This is a white powder coated aluminium edging used to seal the joins of the exterior of your RV e.g. where the side wall meets the roof or the rear panel. (Has a 25mm lip to cover the roof or front/ rear).
The aluminium has a channel running through the centre designed for a plastic moulding to be inserted to hide screws.
Trueline aluminium is also known as" Millard Series 80 Aluminium".