At the moment Aldi have digital bathroom scales for $9.99 can weigh up to 180kg's so no good for the larger van, these have a glass top so would probably need a piece of ply to spread the load, but with a bit of 2 by 4 timber under the tow ball receiver and lowered down onto the scales, hey presto you know the weight on your tow ball.
Seen it done with bar . Brick on one end scales on the other with weight in the middle . You double what scales read..
Or proportionate to ratio of where the weight is on bar .. So scales can take the weight..