Not an expert on these, but i had adblue issues with my Q5 FY recently, so much so that i did alot of research into the system and how it works. On the Q5 at least, there is a non return valve on the tank entrance - so you can't really just suck it out somehow.
However, there was a drain plug underneath. Hopefully, VW has this on the T6
as well.... Basically, you need to drop the tank and find the drain plug. The system will do a check consistently to check the dilution level (i think 33% but can't remember), you can check this with VCDS/OBD11 as well by reading the data values.