According to the AAMVA CDL Manual, what is the most important item to check for during a tank vehicle's pre-trip inspection?

AAMVA CDL Manual §8.1.1 (p. 8-8-1)

Tank vehicles carry their cargo as liquid or gas under the body of the truck, so a leak is both a spill hazard and often a crime to drive with. Making leak-checking the first priority of a tanker pre-trip reflects how much more consequential a tank leak is compared to many other inspection items.