I recommended a CTEK in that thread as the original install had already been fitted with a CTEK to better charge the AGM battery it was just an old model that didn't have lithium capabilities. So a straight swap made sense as all the cables and fuses were the same.
You'll need to disable the split charge relay in the Sargent (or use it simply to signal the DC-DC charger that the ignition is on) and also the built in "charger"
For a new install I'd look more at a 30A charger, the Victron one is popular but runs hot, the Renogy 50A downrated to 30A in the app also and there is the brand new Victron just coming available that
@Dellmassive has under test.
If you are replacing cabling make sure you can get the proper high current crimps done, you do not want hot spots behind your furniture.
A 100Ah LiFePo will give you roughly twice the usable capacity and Roamer/Fogstar/Renogy are brands often talked about. You have space (but you will need that for new chargers).so you could go a bit larger on the battery.
Realistically with a good compressor fridge you may get up to 3 days off grid with no charging. To get beyond that you'll either need to do some driving (a 30A charge will refill an 80% discharged 100Ah battery in about 2.5 hours) or have solar that will keep up with your power usage.
What you need to do is some boring planning before spending. Sit down and estimate what powered things you will use and for how much each day to work out your daily power budget.
So to work out your "power budget" make a list of the things you need to power, how much current (amps) they draw and how many hours they will be on per day - add up all the results and that will give you an Ah figure.
For a compressor fridge if we assumed 5A when running so for a day a rough estimate would be:
5A x 0.4 x 24 = 48Ah
You now see why everyone talks about fridges. Anything to do with heat (either heating up or cooling down) uses a lot of energy. And why there are so many tips on maximising run time by pre cooling and maybe freezing some items.
I've seen folks mention figures of fridges running about a quarter of the time which results in a slightly better figure of 30Ah - but lighting loads are going to be much lower and the fridge will dominate the calculations so maybe use a more worst case figure.
I think if you can add solar that will increase your runtimes - a nominal 100w panel is going to put in around 5A in good sun, so an 8 hour day may replace all the fridge used.
