i want to use the shunt to measure the AGM leisure battery, but all of the load from it uses a grounding point at the rear of my van not directly off the battery.
do i use point A the negative off the battery to the van battery or point B the negative off the buz bar where all the load is?
As stated by @pwfarnell, Point A is correct. However, I think your diagram is incomplete.
Keep in mind that ALL connections must go through the shunt. You cannot connect anything else except the shunt to the battery. If you do connect anything else (charge or load) to the battery then the readings provided by the shunt are no longer correct.