What technology used to measure the thickness of a closed tank like LPG tank?

Ultrasound. The machine sends an ultrasonic tone into the tank wall, and picks up the return sound (vibration) that reflects off the inside surface. Knowing the speed of sound in the wall material and the time it takes to come back will tell you the thickness. You can use the same technology to check for cracks and other deterioration, since that will throw off the return.