Measure the panels on the side of the trailer - most use 48" wide panels, because of this you can set your printed panel width to give you overlap but then trim at the end of each panel to give a seamless look. Almost no reason for seams if you plan correctly. Use an air release vinyl, can be polymeric calendared to save money or cast if you prefer. I would use a good UV Wrap Laminate on this. As example, Oracal 3551 Rapid Air is a polymeric calendared perfect for a case like this, topped with OraGuard Gloss 290 cast wrap laminate will have a 7 year warranty.