Tow ball weight.

Park the trailer/caravan on a level surface. Place the scales under the drawbar with a piece of wood on top to spread the load and jack the trailer off the towing vehicle with the jack sitting on the wood on the scales and the reading on the scales is the tow ball weight. If the scales don't go read enough, get a short plank, put on ond on the scales and the other on a brick. Place the tow ball load (via the jack or jockey wheel) exactly half way along the plank and the just double the reading on the scales. If you are near a weighbridge another way is to weigh the trailer with the jockey wheel on the w/bridge, then weigh it again with the jockey wheel off the w/bridge. That way you get the ATM and GTM and the difference is the ball weight.

Illustration here.

http://caravanersforum.com/viewtopic.php?f=60&t=6557&p=83572&hilit=bathroom#p83572

Hi Rob,

Like this

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Cheers, Bruce.

Could always try a sliding scale, or bag scale as some call them.
Just hook it on to the coupling and lift. Available from places like mitre 10, but not bunnings. About 20bucks.