Hi @tatt1ben,
The first thing you'll want to do is confirm with the manufacturer of the shed that they are comfortable with you hanging a TV from the frame of your shed. They will be the best people to give you a yes or no based on the quality and calibre of materials used.
From there, the usual process for hanging TVs when you do not have a stud directly available is to attach some plywood between the framing members, in your case, the horizontal girts. You would then attach the TV mount to that plywood.
As long as the manufacturer gives the go-ahead, I would say a sheet of 18mm plywood attached to the two girts with hex head metal screws would be the way to go.
Let me know if you have any further questions.
Jacob
