Nice. This is similar to how I have my reef set up at home. Couple of thoughts for you:
1. Each drum, when full, will weigh about 126 lbs. Be extra careful with how you set up the shelving!
2. As a failsafe, drill a hole in the side of each near the top, screw in a 1/2" fitting, and attach some 1/2" tubing that goes to a drain. Overflows are no fun.
3. Remember that if you use a float with an adjustable arm, you can mount these in the top of the container.
4. For the container that you use for top off water, suggest that you never put more water in there than about 110% of the amount you'll need each week. So if you have that tank gravity feed a float valve in your sump, and if that float valve ever fails, there will be a limit to the amount of fresh water that can enter the sump. So after you fill that tank once a week, close the ball valve that feeds it so its not constantly re-filled.
5. You want to set up the RO
system so that it comes on at least once a week, and so that it doesn't constantly short cycle (as it will if you have it connected (indirectly) to a float valve in your sump.
6. Suggest you use 1/4" tubing and quick connect fittings (including 1/4" bulkheads) rather than PVC
to route water from your RO
system to the tanks.
7. If you want to automate it more, you could add an irrigation (garden hose) timer so that the system come on once a week for x hours.