Trying to describe this isn't easy, but its not that hard once you look at it. Say you measured the angle of this tube from the front plane and side plane of the vehicle. You could do a 2d sketch on the front plane showing the angle of the sketch. Endpoint 1 could be the origin. Endpoint 2 could be somewhere off in 2d space. Your second sketch would need to be on a plane intersecting Endpoint 2, but parallel to the side plane now. Sketch a line starting at the origin again, but using the angle you measured from the side of the vehicle this time. Where ever that line crosses over the sketch Endpoint 2 from you first sketch is your new Endpoint 3. Based on the two sketches in two different planes, Endpoint 3 will be a fixed spot off in 3D space now.
With the two points you can now form a line between them, although I am wrong in that you'd have to do a 3rd sketch and depending how onshape works you might have to do more to get that into a line. If it allows you to 3D sketch simply connect the dots and start extruding your tube. If your stuck only using 2D sketches then you have to figure out how to get the two endpoints you need onto a plane that you can sketch on, and that might require a third sketch on the top plane.
You could do the whole thing in one 3D sketch if you wanted to, but you'd have to figure out a way to convert your angle measurements into more of a coordinate system measurements. Measuring that on the vehicle would be hard and I suspect it would be easier to break into two different measurements from different vantage points.
I wouldn't say there is a right or wrong way to do this type of stuff in cad, its more about what you can do and measure with the software and capabilities that you have and be confident in your results.