Hi Sarah - I get leg cramps anyway, not too severe but enough to keep me awake sometimes at night.
Banana's probably help cos they are quite high in potassium, which is similar to calcium and sodium (i think from rudimentary chemistry...) They commonly form salts?
My suggestion would be to massage with something like deep heat (check if safe during pg) before sleeping and try wheatbags/icepacks on the legs when they cramp up.
I've always found massage to be the best thing for cramping.
Hope ya get better!
Fi