I am not a fan of using an RO unit for top off "directly".
If you have a RO unit with a TDS built in, then check it when you activate it check in and then out. At least my water supply, when it first starts up my TDS are up at over 100 in, and 0 out. So the DI is doing the job to get rid of the TDS, but the starting and stopping of the RO Unit for small batches will BURN up your DI resin in no time.
When i run my RO for a few minutes the TDS in will drop to about 6-8. So large batches of water actually conserve the DI in my case.
And what is the worst that can happen? Well if for any reason the float fails then the RO unit will produce water infinitely until you see it, then overflowing your sump and reducing your salinity.
So if you decide to do it, you need to have a second float to deactivate the solenoid.
It is probably just easier and safer to run a standard Top off, meaning a small reservoir and a pump with the float switch.
Just my opinion....