36 convertible electrical issue

New to forum. I just purchased a 2004 36 convertible. Has 3126 cats and having electrical problems.
1. Port engine started intermittently giving start up issues. Now it's always. When in run position voltage gauge shows 9 to 10 volts and clicks when trying to start. Checked batteries they are a year old and show 1275 cca when tested at two separate places. Cleaned connections and still doing same problem. I tried using parallel with starboard running and still not enough. I jumped batteries together and then use parallel and it will start. Next I'm going to check connections to ground from battery but I'm hanging hard time tracing ground path from battery because they go under generator and I can't see behind. Any suggestions?
2. Other issue is with AC and DC panel when the battery charger is on port side dc panel works fine. If you shut Ac load off it kills dc side. Again batteries were both tested and are good. I didn't think Ac side and DC were connected in any way?

West marine used the little tester handheld. I didn't trust it so went to O'Reilly's and they used the larger bench tester both came back good. I'm doing ohm on ground starting from batteries to engine. It's very hard to see because from batteries it goes under generator and I'm having hard time following. I looked at wire schematic and it shows from batteries to engine. I would like to know which ground goes directly back to battery because I see one above the oil pan on block. And another in bell housing by transmission going to back of starter?

It's almost embarrassing to post the solution that fixed everything. I started ohm testing and tracing from starter to grounding block to batteries and everything checked out. I put the volt meter on back of starter and had 12.7 volts and turned starter switch into on position and voltage dropped to 5 volts. Luckily I heard something arching in the electric box above starter. I had to disassemble couch to lift hatches so I could lift lid of electric box on engine. Then I went back to starter switch and when put into on position voltage on back of starter only dropped 1 volt. It started right up and no problem. On the back of the lid of electric box was a laminated wire schematic folded in half and plastic had melted and that was squishing wires in electric box and causing a short. Put the lid back motor fires up with no problems. Everything runs perfect. If you have 3126 cats check the electric box I don't know if caterpillar put this in or previous owners mechanic but it was the problem.

One other question what size battery should be on each motor? The previous owner has 8d 1335 cca from Napa which are not marine batteries they are truck batteries. I'm asking because I ran the windlass and it drained the batteries causing the parallel to be needed to restart. Once I ran charger for hour it charged it back.

Read below my fix of the problem you'll get a good laugh. Previous owner had two 8 d 1335 cca from Napa. These are truck batteries and not marine. I've read some different specs but would appreciate if anyone knows for sure what cca batteries should be used? Of course I'm replacing with marine 8d.

Capt j I ran it for fifteen seconds. Then was to close to another boat picked back up for another 15 seconds and dropped it back for second time but I couldn't bring up battery would drop down to 9 volts so I manually pulled it up. Charged it for about an hour and I can start motors but I'm thinking the 8d truck battery from Napa doesn't have same amp hour reserve that a marine battery has.

yes checked water level it both batteries were good
I don't know what kind of idiot sets and pulls an anchor without engines running but that's not me. The drop in voltage when I pulled it up the second time caused port main to shut down and then I had to parallel to restart. Pulled anchor up by hand. When I got back to dock an hour after with charger running re charged battery.

Agreed this is the first boat I've owned that runs house through motor battery. I'm going to check alternator output.