Since you're talking about batteries and inverters (something near and dear to my heart since my first Heart Interface 1200 & 2500 watt inverters in the '80's), just for your consideration, when I installed the engine mounted trolling motor on our CVX-20, I wanted to put it on a separate battery just for ease of mind. The previous owner had put in a second battery for the stereo, but didn't provide any way for it to charge without just hooking on a car charger.
I considered diode type isolators like I used to use on the campers, but I wanted to keep things simpler for an outboard boat since they don't have a separate alternator in my case. I had purchased just a simple on-off battery switch and was just going to connect the two manually while the engine is running as well as mounting a mini-charger that I could simply plug in while at home.
Before I managed to get the switch installed (like that very day), I went for parts and happen to notice in the battery section of Fleet Farm, but in the solar regulator portion where I hadn't paid any attention to while searching for the switch, this little box for connecting two batteries like in a typical boat situation (not a cross 12v-24v trolling motor battery bank situation - that's a whole different animal). It's simply connected between the batteries just like I planned with the switch, but it will only charge after the first battery (starting and main battery in my case) comes up to enough voltage. It's just a mini version of the "smart" type of relays (actually old technology with newer monitor controlling) that they are using in the newer motorhomes. Also has a push button to connect the two batteries together in case you needed some quick starting boost (in theory).
Anyway, it's been working great, has been charging the second or aux battery while running the engine and/or plug-in battery maintainer. Was only around $65, and doesn't have the hassles of the voltage drop and extra hookup of a diode type isolator. So just mentioning in case it would fit your situation.
The only caution I would add is that I think it's 100 amps or less, which was perfect for my situation, but if you are charging from a high output engine alternator a battery bank for a 1200 to 3500 watt inverter or larger - then it's definitely not enough capacity to consider.
Side note: I know you guys know what you're doing, so I'm not trying to lecture in any way, and I know the cabin cruisers and houseboats are equipped with modern conveniences all the time with strict wiring standards, but on a personal level even though I love the technology - inverters and even regular dock hookups scare the "bajeebers" out of me from the standpoint that I grew up on a farm where we had three-phase high voltage power that had a tendency to manage to find (sneak) it's way around almost any water-proofing standards during wet conditions eventually. I know it's not quite the same, and not trying to scare as much as to caution, but earlier this year I heard of two incidences of electrocution near a boat and also near a dock - in both situations if I remember correctly it killed multiple kids and people, and I never heard the final report, but they were expecting to find worn and/or insulation issues from the dock incident. Just something to always keep in mind.
