I like to mount antennas on the back of the cab. Especially on the cross bar that holds the spring and air lines. It provides a ground and is up and out of the way.
No it doesn't. It provides a DC ground but it doesn't provide a RF ground, the two are not the same. Ground for antennas depends entirely on the metal directly underneath it, not alongside it. Your groundplane is effectively the width and length of the cross bar.