Always start at the source of power. Multiple batterys have multiple connections. Multiple places for connections to get loose or crud developing in them.
We are so quick to blame something, expensive, else when it is usually a simple fix.
Boat: 32' 1996 Albin 32+2, single Cummins 315hp
40+ night per year overnighter
2007 Alpenlite 34RLR
2006 Chevy 3500 LT, CC,LB 6.6L Diesel
Ham Radio: VP9KL, IRLP node 7995