To the best of my understanding - if you want to run an auto cut off from your RO directly to the ATO reservoir, you need a solenoid and then an ATO float switch in your sump to signal the need for the water to move from the 5.5g to your tank.
Are you using gravity or a pump to move the water?
You can get an mechanical float switch if you're using gravity. If you are going to use a pump - you can get any type of ATO that uses a float switch to power an outlet. I'd suggest one that uses a snail guard and has a backup float... because wet floors are not very fun.
I bought all the pieces to try to make this happen, just haven't had time to do it and test my theory out yet.
Hm good idea about the solenoid and float switch, didn't think about that.
As far as pumping water from the ATO tank to the sump, i just use a pump, to pump to it. Digital aquatics makes a dual float switch kit that hooks up to the RKL so ill be using that once i get money.
any idea where i get this solenoid to do what i want?