About the 6k mark repeatedly for people has meant one of two things. Either they did not truly fill up both of your tanks even though your paper work says they did OR the transfer mechanism between both tanks is messed up. It of course could mean your car is consuming massive amounts of DEF but I do not think a single person on here has actually had that happen. Be a bigger jerk to the SA, that is what I did when this happened on my car. Chances are they'd never admit to not filling both tanks and you can't prove it either way unless you try to top off both your tanks after they claim to have filled them.
--Admiral Aaron Rouse