The solenoid should go after the ro outlet and before the float valve. BTW - when I write float valve, I'm talking about a device which physically stops water from flowing. A float switch is a device that causes electrical contacts to open or close.
If you're not using an auto-shutoff valve, I would recommend that you purchase one. The auto-shutoff valve, a float valve, and a 1/4" check valve will allow your RO system to fill your container. When the container is full, the float valve will shut. That puts pressure on the auto-shutoff valve - that stops your system from allowing water to flow through it.
1 - Float Valve ($10 to $12)
2 - Auto-Shutoff Valve ($7 to $8)
3 - 1/4" Check Valve ($3 to $6)
Check to see if you already have an auto-shutoff valve installed. If you do, then you may already have a check valve.
What you are trying to do with your solenoid is necessary because you want to automate the act of starting the container filling when it is low on water. But before you get there, you should have the auto-shutoff valve and float valve (above). They will function as a necessary back up to your Top-Off solenoid.
If this ro system stuff is new to you, just google "ro auto shut off valve diagram". You'll see lots of pics that might help you. On the solenoid front; trying to automate a top-off function with only one float switch is not practical because water surface movement will cause your float switch to burn out due multiple "switches" per second. You'll need two float switches - one for high water and one for low. If you already have an auto-topoff for your sump, that's what we are talking about.
My RO container is a 55gal drum. It has a float valve for backup and two float switches. One switch is at the top of the drum (where I want the max level to be) and the other is about 6" from the bottom of the drum - it start the fill phase.
