Chris, I have been reading your posts with interest, because we have a very similar situation to you (caravan, coffee machine, 1800 watt inverter, microwave, 200 amp hr AGM's, 280 watt solar) and replaced our batteries in June after 18 months use (totally fortunate the supplier decided they should have coped and replaced under warranty). I researched the lithium option because weight was also a big issue. I wasn't a forum member then but separately came to pretty much the same conclusions as you, although decided to wait and see how it developed as time was on my side with the new batteries. Summer has never been a problem, only winter. I am reluctantly looking at a generator, which I won't be able to use in many places I go to, but ultimately I see lithium batteries as the future. So please keep the posts going, your experience in the next year will give a great insight.
Hi Justintime, one quite important thing that could be the cause of an early demise of your 200 amp /hr AGM battery bank is the excessive load put on them when operating the coffee machine. With the monitor we have now I regularly witness up 125 amps being drawn from the battery while the coffee is being brewed. This high drain is only for a max of about 1.5 minutes and has no detrimental effect on the Lithium battery, but research has led me to believe that even for a short duration, AGM batteries cannot tolerate that high current drain. On an American forum they were recommending C5 which equates to one fifth of the batteries capacity, which would allow only a current draw of 40 Amps from a 200 Amp Hr AGM battery bank.