Finally ran my air vent for the front grey tank.
It runs up inside the body, and exits near the roof.
I used some pex tubing and unions that were salvaged from the previous truck.
As it exits out the top, I used a 90 degree union.
Cut a hole in the wall, ran the threaded part outside, and cut some screen to go over the hole, and put on the fitting.
Bead of silicone, and we were done.