So, lets spec out and set up a system of LiFePo4 batteries and inverter

[Sunking]

I did not explain myself clearly enough. What I was trying to propose is that with AGM batteries like Lifeline that can take a high current, if a grid-tied charger were chosen that puts out 0.3C of the 20 hour AH capacity, when the battery degrades to 80% of that capacity, it can still take the same current (unlike flooded). Perhaps a hybrid golf-cart battery that can also take a higher amperage could also be used in this way.

Absolutely it will still take as high of a charge current as you can pump into it. It will just go nto Absorb a lot sooner than it should then take longer to recharge. Sounds crazy I know, but that has to do with the increased internal resistance of an aging battery.

The final sign the battery is done is when the battery cannot supply a load for very long, and when you put a charge on it, your charger gets fooled into thinking the battery is fully charged and shuts off or goes to Float very quickly. Its over.