Many if not most tank trucks have pumps that draw a vacume on the tank. The same 3 or 4 inch hose that lets liquid out the bottom of the truck can be used to "suck" liquid up into the truck.
I just asked something about that.....dang....guess I am not good terrorist material, the convention would be over by the time I figured it out...lol.
And while they do "gravity feed" when being off loaded... you need to understand that most petro chemicals they haul have a certain pressure on the tank. Like drip gas from wells might have 30 or 40 pounds of pressure on the tank. As propane would have 200 to 250 pounds of pressure on the tank. So it's not just a gravity feed when they offload.