I more than likely don't understand but why would you want to weigh your tug rear axle with the van hooked up? This won't give you a ball weight as the weight of the vehicle will be in the mix wouldn't it?
Yes, it wont give you the ball weight, but there's 6 weights you benefit from knowing, and can be measured in this order (best done with all passengers food and fuel on board):
1 Front axle weight - more recent vehicles than any of mine have this weight specified, and this reflects the load on there with the weight of the trailer and rear load in. Too light and you poor tyre contact with the ground and therefore rubbish steering, and if IFS rubbish tyre wear on the ouside of the tyre as that is the only part in contact with the ground.
2 GVM of the tow vehicle = weight of the vehicle, including the load from the towball transferred to the vehicle
3 Towball weight = the weight on the towball (pull nose of trailer on to weigh bridge and put it on the jockey wheel, take vehicle of weighbridge)
4 Train weight or GCM = the weight of the loaded tow vehicle and trailer. Manufacturers specify this
5 Rear axle weight = the load on the rear axle, including the towball weight that is transferred to the vehicle. Again manufacturers specify this on more recent things. May need golook at moving weight forward to get it under the limit if theres capacity on the front axle.
6 Trailer GVM = the total weight of the trailer