Disconnect Solar Before Checking Battery Levels?

You can check water level anytime, but you should have the batts fully charged before topping up so they are at their highest level before you add any. How can they be at their highest levels unless they are being charged? Observe the usual precautions against acid splatter.

OTOH, you want to keep the plates covered when the batts are resting and not being charged, so you need to check levels then too.