If you achieve one half percent amp hour capacity amperage at 14.4 volts the Lifelines are charged. Work on that goal. There is zero reason I can get 10 years out of a Lifeline and you only get five all things being equal.
Shallower cycles, high amperage recharges and full charge once a week will be the Fountain of Youth for any AGM...and a hell of a lot cheaper.
The most profound condolences to you and your's about your loss. Watching parent's, siblings, and finally friends, passing, has to be the most detestable part of growing old. These days I am almost afraid to re-establish contact with friends that have gone unheard from for many months. I usually have a sarcastic bit of humor for most things but this isn't one of them... :(