yes, you would have to remove the tank to weigh it, as well as to inspect it. That is also my idea of the -correct- method, shutting off the gas, running the lines dry, checking to be sure the line is dry before disconnection from the truck, wearing proper face, arm and hand protection, removing the tank, filling it to weight on the scale and checking the inspection information each time,
However there are many companies around that use on the vehicle filling, and/or use "80% from the spit valve flow is full" and to begin to insist that they are _wrong_ is also about equally "incorrect". [ymmv]