The air vent on my fresh water tank is just a hose attached at the top and routed down under the floor. It doubles as the overflow as well.
Black/grey tanks aren't vented directly, but the plumbing attached to them is, just like on a house. Up and through the roof.
As long as you have SOME way for air to get in/out of the tanks from the OUTSIDE of the vehicle (don't want smell tank air in your truck...) it should work ok. Building it to code for a food truck may be a different animal.