Common sense would tell you if you are purchasing an expensive trailer , now knowing about weight issues across the board, that a weighbridge certificate at point of sale, whether it had full tanks or gas bottles wouldn't matter as they are easily added, would be a priority for any buyer !!
Jayco have been shortchanging people on their weights for years, but they all come up with a figure for a trailer in "basic" spec. That may not even be in a condition to be towed out of the factory, but a weight when it sits in the showroom ready for you to take it home should be compulsory, but we know that won't happen.