Hi, a few options come to mind.

1. Keep the things completely separate. Retain the existing panel for the current set up and a new panel, regulator & battery for the fridge.

2. Use a 2 way switch. When bank 1 is full (on your voltmeter) switch to bank 2.

3. Connect the 3 in parallel to make one big bank. Doesn't really meet your requirements.

4. Why the resistance to a VSR? We've used one on Alibi for 2 years without a hitch. Dead easy to wire up. Panel to regulator, reg to bat bank 1. Vsr between banks.

5. Use a dual battery regulator.

://www.solarcampingaustralia.com.au/dual-battery-c-35_62/20amp-12v24v-dual-battery-solar-charge-controller-p-251.html

www.redarc.com.au/20a-solar-regulator

www.autoelec.com.au/ctek-d250sa-dual-battery-smart-charger-12-volt-sol

And loads more on eBay etc


I reckon 5 would do the job well.

Cheers

Bristol

Thanks Bristol & Cisco.
further to your points Bristol...
1, I like this but as you know real estate is at a premium on a small boat, but I could bolt on say a 150w panel next to the 50 w panel I suppose, it might look a bit hotch potch but would work. I also thought the day might come where the 50w panel doesn't keep up with all the other house loads (except fridge) but that day hasn't come yet.
2, doesn't sound foolproof, not good for me.
3, yeah doesn't suit the different types, sizes and vintages of batteries I have.
4, I'm trying to avoid the possible failure of one, if I don't have one it can't fail. Plus my understanding of them is if for example the fridge is on battery 1 and it's in constant use, then the voltage may not get high enough that the other battery banks never get to receive a charge???
5, yes I like this point too, I currently have a "duo" type regulator at the moment and it does a stellar job of keeping the existing battery banks charged and separate.
How would you add another bank? Have another duo wired between the existing duo and splitting off to charge banks 2 and 3?

a diode sounds like it would work, I'd need to find the correct one,
cheers

Ah, how about a triple regulator?

www.google.com.au/url?sa=t&source=web&rct=j&url=https://www.energymatters.com.au/ampair100-regulator-for-12volt-triple-battery-bank-p-660.html&ved=2ahUKEwjmg__FtKrbAhXFgbwKHRcyACMQFjAAegQIBxAB&usg=AOvVaw3EpFaOZGE4Ed3Jm9fcntEC

I can't see any reason why you can't simply split the feed from the new panel. One feed to your existing dual controller and the other to a single battery regulator and then the new battery fridge.

Btw, we use a 50w for all power ( no fridge ) and it has been fine so far.

Cheers

Bristol