I have an RO unit with a deionizer. I have a TDS meter that measures both the TDS after the RO membrane and after the DI unit. My water after the DI is 0 TDS. I just measured my pH, and it is 6.4 sitting in the 30 gallon container. It has been sitting in the container for almost a week.
I agree that something is amiss if your pH is 8.
It sounds like your RO membrane isn't filtering too well. The membranes do get clogged up over time and depending on the water it is filtering. It is recommended that 70 psi be used to push water through the membrane, so I have a pump that creates the pressure for water to enter the unit. You may want to talk to an expert at an RO manufacturing business about your RO water.
I recommend talking to this company: AquaFX 407-599-2123
https://www.aquariumwaterfilters.com/