One item my dealers head mechanic recommended. If you are going to use the on board system for charging, because of the greater capacity (whether 2 6v or a really big 12 v) the installed wiring/fuse systems may not be able to handle the initial current sent to charge deeply discharged batteries. A fuse will blow. They recommended to replace the charge line fuse with a self resetting circuit breaker. It may be a little slower to get everything up to 100%, but you don't have to replace the wiring, etc. to accommodate the additional current.