What is the tds going into the tank and how are you reading it?
If you have an inline meter, is it giving you the correct reading? What I'm saying is that you need to use the same meter to test the water in and out of the holding tank. Is the hand held meter reading correctly. (At zero tds, it would appear that you are polishing the water off before storage as no r/o filter will work 100% efficiently.)
If the water in is fine but the water in your holding tank is contaminating the water out, then you have the answer as to what to do - clean out the holding tank. If the water out is fine then the problem is something to do with the van tank. Do a step by step analysis of the water on it's route to the brush head to identify where the problem is?
You can still work comfortably with a tds of 4.
Long term, everyone should have a bottle of tds test solution to check that their tds meters are reasonably accurate.