The correct way to fill tanks is by weight. So, your filler did the correct thing. BUT, lets say he was NOT knowledgeable as to how to fill by weight. He used too little weight OR he failed to use counter weights for a 30 lb tank and used the weights for a 20 lb tank. Called Tare Weight which is listed on the collar of DOT LP tanks. OR they were crooks and did not fill full but charged for it. With all 3 tanks empty, you can verify the fill. They should hold 6.8 to 7 gallons of liquid LP. Doug
PS Filling by weight means you STILL open the 80% valve. IF Liquid spews before the weight is matched, you shut off the fill.