What should a driver verify about manhole covers and vents during a tank vehicle inspection?

AAMVA CDL Manual §8.1.1, p.8-8-1

A gasket that's missing or a cover that isn't seated correctly can leak cargo along the entire route, not just at one spot on the tank shell. Clear, working vents also prevent dangerous pressure buildup inside the tank, so this check protects against both spills and structural failure.