I'm not an expert, but this is how we do it.

The tankers we've had all had some kind of 2-4" NPT fitting on the top somewhere to deal with. Some had special pressure relief fittings. However, I remove those fittings and install a 2" NPT hose barb fitting, then run 2" suction kan-flex hose down the side of the tank, and then terminate it with a ball valve at the bottom near the load/offload point. We have a vinyl sign made saying "VENT TANK PRIOR TO LOAD/UNLOAD". Obviously, if you load thru the hatch on the top, it's not necessary to vent the tank. But bottom loading or unloading requires venting. The ball valve arrangement allows this with 100% chance of no back pressure. A 2" hose/fittings seems more than sufficient to handle venting from loading with any of the big terminal hi-flow pumps we've encountered.

Having this valve at the bottom allows for any splash or overfill to come out the bottom of the valve near the ground so it doesn't splatter all over the tank. Sometimes the catch well at the top (surrounding the hatch) also has a drain built in that needs a hose coming down to the ground also. But that has nothing to do w/venting the tank.