We were unable to locate specifically where the vent line from the rear routes around the right motor mount. We crawled under one of the M998s on the ranch and were able to snake a cell phone up to take some pictures.
As best as we can tell, there is simply no cushion clamp to mount the vent line, and it lays across the frame between the motor mount and the body mount (just under the mount flange riveted to the body).
Viewed from the rear of the RH front body mount, it is clear that the rear vent line leaves the top of the frame to lay across the frame support for the body mount. Also visible is the fuel tank vent arcing upward underneath the fuel line support and cushion clamps.