I feel the OP's pain, as I did the same thing a ways back. The sun had bleached the red positive cable so that it looked white, and all it took was a split second of not thinking to blow the fuse. I had packed spare fuses, but unfortunately I didn't have one that big. The RV park's store didn't have anything that big either. We spent an evening in the dark and the next morning I drove 60 miles to get a fuse.
I'm not sure that an EMS would have helped in this case, as they generally protect against faults from the shore power supply. I know they can be wired to provide protection for systems with generators. Question for you electrical gurus with generators. Would an EMS wired as such have protected against a reverse polarity situation with the battery?