a motorized pump and a garden hose would work very easily
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.
A pump and garden hose at an average of 2 gallons per minute would take over 76 hours to fill the truck.
The best way to get a big explosion, though would not have the truck full of gas, mostly fumes. I've heard the fumes of 1 gallon of gasoline have the explosive power of 8 sticks of dynamite in the right air/fuel mixture.