Many of us have seen our DOT tanks (not the big horizontal tanks, but the tanks which are the 30 or 40 lb portable tanks that most RV's use) being filled by either by weight or by volume. Seen them done by both methods many times.
When the tank is filled by weight, I have NEVER EVER seen anyone compensate for the weight of the LPG hose and LPG contained in the hose when filling the tanks. And guess who pays for the weight of the hose and the propane contained therein??
Might watch to see what happens when your tanks are filled by weight.