Parallel is no good. Will not charge or discharge evenly. No matter how you have the wire connected.
Please give us the details & scientific dissertation on this claim. Please keep in mind that there are millions of batteries out there wired in parallel, it is one method of building capacity. This is done not only in marine applications but in industry, fire and EMS service vehicles, uninterrupted power supply systems, cordless power tools, solar, military etc. etc. on and on. Even some of the large truck makers have moved away from 4D and 8D's for engine starting and are now wiring TPPL AGM batteries, often group 31's in parallel. I have some parallel banks out there at 13 years of age still performing well. I also own the equipment to test batteries via internal resistance and well wired parallel banks survive and charge perfectly fine. If they are "single end wired" I do see imbalance issues and my test equipment shows me this is possible. When end to end wired I don't see imbalance issues any more so that with 6V batteries... I generally prefer 6V or 2V batteries but even with 6V you will need to wire series parallel to gain Ah capacity. I have some vesels I work on with banks in excess of 1200Ah. You don't get there on a boat, with the space constraints, without using parallel wiring..
Sure it would be nice if we all used 2V cells in series but they are huge, expensive and tough to fit. They represent about 2% of the banks I see on boats. When you jump up to 6V you lose many of the benefits you'd gain by using 2V cells. No longer can you measure individual cell voltages as you can with a 2V bank because the individual cells are not accessible for voltage measurements..
That is not a combiner it is an isolator. Again please explain for us scientifically why you "need" a combiner to charge evenly.
I suspect the OP simply got a bad battery, it happens. Another possibility is these are private labeled "battery distributor" batteries where they slap labels on anything that comes through the door. I had a customer two years ago with identical labeled/stickered batteries yet the batteries were completely different brands and not even a similar case.. Differnt brands use water at differnt rates. Also the battery losing water could be the battery closest to the engine and running at a higher temp. I have even seen batteries get heated by dark colored topsides when the sun comes up. The SG should be checked to confirm there is not a cell going bad...