Actually, there's something you can do besides a reservoir. You can hook your RO/DI up to a sprinkler timer and time it so that it only goes on for about an hour or two at night. That way, it will only fill during that period. Kind of not a perfect solution because once the sump is refilled it will still do that on/off/on/off thing but at least it's not doing it all day.
The reservoir idea is easier and cheaper
a 5g bucket if nothing else, will work fine, or a rubbermaid, or whatever you can make work.