Help with battery wiring please

Hi. We have a truck camper with a two aux batteries in the camper battery box. It has a 4 gauge charging wire about 8 feet long from the truck alternator. I am planning to move the batteries to a new location and will need to extend the length of the charge and ground wires about 3 feet. My question is what would be the best method to make a good connection to extend the wires with minimal resistance thru the splice. Would it be better to replace the entire wire and make it one piece and possibaly up grade to a larger wire at the same time? I am thinking eleven feet for 4 gauge wire may be pushing the limit on voltage drop.
Thanks for your help R.