Have you considered putting your second trolling battery where your starting battery is? Then both trolling batteries would be together, your charger would stay in place, and your trolling motor battery harness would stay the same. Then you could mount your starting battery up front. Go to NAPA or a welding supply and buy good heavy copper welding leads to run your starting battery current back to a junction block. At the junction block, tie the starting battery leads, engine leads, and the other circuits which were on the starting battery all together. If your charger does 3 batteries, hook that in to. When you buy the welding lead wire, pick up the lugs for the connections. I like to solder mine, rather than crimp them. If you can't find a suitable junction block, you can easily make one with some brass bolts and heavy plastic for insulating. PS Make sure you buy some red electrical tape so you can identify positive from negative when installing the new starting battery leads.