+1 to what Paul and Erik said.
I do new SSB installations professionally, and a lot of troubleshooting of existing ones. From my experience, the lack of a proper power cable setup is the cause of many problems.
For one-way runs up to 20' you need to use AWG6 cable from the house batteries to a terminal strip located near the radio. Then, cut the OEM power cable which comes with the radio to a short length to connect at the terminal with the larger cable. Leave a bit of extra. If you like, use a good switch like the Blue Sea Systems #6006 in the positive cable near the terminal switch.
Power for the tuner, if needed (as with the SG-230 coupler) can be taken from this same terminal strip. And, if desired, the VHF
can also be powered here.
What's not optional is to install a 30-40A fuse in BOTH the positive and negative cables
, located as close to the batteries as possible. There are only two types of fuses
which meet ABYC requirements for direct connection to large batteries: ANLs and MRBFs (the new terminal fuses). Class-T fuses are OK, too, but are too large for this application. I prefer 40A ANL fuses and holders.
Note that circuit breakers are generally NOT compliant with ABYC recommendations, except for the new high AIC Blue Sea Systems type.
Attention to this detail will save a lot of grief, both in getting clean power to the radio and in reduced RFI. Both ways: from the radio into other equipment
and devices on the boat, and from these devices and equipment
back into the radio.