Putting a storage/surge system into the grid is becoming necessary because of the variable nature of renewable energy sources. As someone else mentioned already, much like the need for batteries in your RV solar system, even if you only need power in the daylight hours. Solar panel output can vary rapidly due to clouds etc., With the issues/costs involved in making large generating systems able to handle larger short term fluctuations it makes sense to buffer it with something like large battery banks.
Regarding the issue of using water as an energy storage medium, Here in Utah we had a couple projects where they tried to use the method of pumping water to higher levels during oversupply, and using it to run generators during peak demand. For better or worse neither project got off the ground due to lawsuits and pressure from environmental groups. It would take literally decades to get the approval and build one.
Finally, regarding batteries, they're still the weak link in things like solar, electric auto's etc. For many living in dense metropolitan areas, especially ones with moderate climates, the technology is finally getting to the point where EVs are a viable option. For those of us with more demanding requirements, we can hope, but having been involved in battery testing for the military off and on for over years now I'm not sure I'll see it become mainstream for RV's, or long road trips in my lifetime. I first went to school to learn about lithium batteries back in 1981 when the chemistries they were using were so volatile the government made us go through three weeks of training from the manufacture just to maintain and service them.