Turn it off anytime you’re not using the monitor or TV. That will be an easy place to start to reduce your battery usage. That Renogy inverter is too big for your needs and thus uses a lot of battery simply to be powered on. I sold mine and got a smaller Victron instead, which dramatically slashed my battery usage, using the ECO mode.
Sometimes you can get away with using a normal charger to charge lifepo4, but it’s not ideal. I would recommend getting two of these. You can hook up one to each battery to get a total of 60A of charging, which will take 9 hours to go from 0% to 100% for your entire battery bank.
Sorry for the USA link but it gives you an idea:
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The single output model seems hard to find at the moment. Maybe it’s easier in the UK. InvertersRus.com sometimes has used/open box models fora good price but they’d be the North American version. Maybe there’s a good Victron dealer in the UK.
There are cheaper chargers out there but the Victron would be the last you’d buy. Fully programmable and easy to use. I love mine.