My spidey sense is tingling...
I would expect that RODI waste can be plumbed with a 6-7 foot head no problem. I would upsize the tubing if the run is long so the resistance in the line is minimized, so go from the 1/4" I.D. hose to a 3/8" or so. If PVC fittings are too hard to find, use brass, which is fine since the waste line isn't going into the tank.
One idea also would be to mount the RO unit high... by raising it up, you reduce the head pressure, because head pressure is figured from the relative height from the source to the destination, so for example, if you mounted the RO unit 6' from ground level, then the line dropped down to ground level then went up a 6' hill, the head height is 0.
One other idea... don't bother with the drain. Put another one of those giant blue tanks outside your shed, mount it on a short platform, and put a bulkhead and valve at the bottom. Dump your waste into it, then use the valve to fill up watering cans. That's what I do, I reclaim up to 110 gallons of waste at a time, which we use to water plants and such. One day I'll hook a sprinkler pump up to it so I can water the lawn too.
Or, if that's not your bag, dig a big old hole in the ground next to your shed, fill it up with driveway gravel (those 1" or so marble chunks), drop your pipe into it, cover the rocks with a sheet of plastic, and cover the whole thing with dirt. Simple DIY cistern...
Now, I know that my previous conversations with the OP revealed that it "never froze" there, but I STRONGLY recommend, since there is obviously power in the shed, that you install a simple freeze protection heater. They sell heaters specificially for the purpose, they are used in fire sprinkler riser rooms on buildings to prevent the valves from freezing up. Google "Northern Tool" and search their site for PN 173054. I strongly recommend that because if you get a freeze, you'll destroy your RO membrane, possibly fracture pump housings, as well as who knows what pipes and valves. To me, it's cheap insurance.
... besides, you've got a tool box in there, so it must be a workshop, so if you are in there in the winter you can crank it up and at least be a bit more comfortable.
