Something seriously amiss here.
The previous owner replaced the membranes in 2013 just before we bought Elyse.
The system has been running fine ever since. We have an inline TDS monitor and the TDS is still after 7 years less than 300...depending on temperature it can be as low as 150.
We never run the watermaker in the marina, but have a timer and solenoid system that flushes the system with tank water every two days when the boat is left in the marina as it is now, unfortunately.
We never put shore water in the tanks, only RO water .
One thing is that when flushing with fresh water the outlet valve should be all the way open i.e. no pressure across the membrane, and when starting up to make fresh water, the system should be flushed with saltwater for a few minutes to flush out all the fresh water before increasing the pressure to make water. I believe that having high pressure across these membranes with fresh water can ruin them.