I suggest replacing the fuse with an auto reset circuit breaker. It will trip if a short or high current differential occurs between the two batteries. It will then reset and continue to pass current between the batteries. They are available at most auto parts stores, Amazon, Ebay, etc. 10 to 20 amps aught to give plenty of protection.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI