I'll be honest I was a bread and butter sparky not the tech savvy bright young things you see on YouTube in their designer site wear and there genuinely is a difference between sparkies and auto electricians... and I are a bit thick!
AGM batteries are still lead acid and as such will wilt at about 50% of their claimed Amp Hour output meaning you get two days absolute max running that 100Ah AGM powered Waeco fridge in summer when not on hook up, the Lithium though will virtually poop every last drop of electric back out by comparison so on a 100Ah lithium you could run that Waeco for nearly four days.
Lee
@Dellmassive is the man to watch for tech insight and I'm reading and re-reading his Under seat battery install thread to get a better grip on what's got to be done, I will post how it goes especially as I've got most of the bigger bits already, just as a thought Renogy are having a bit of a push at the moment and knocking prices down a bit to the point their 170Ah lithium iron phosphate battery is pretty good value.