It honestly doesn’t feel like 2 months later, but yet here we are. Sorry about the radio silence.
Just an update from my side and some more advice please. 
After using a slightly better multimeter (and just putting some load on and taking measurements) it is pretty easy to tell that one battery definitely went. After roughly an hour with ~250W load it just drops off a cliff voltage wise in a matter of minutes.
To have as cheap as possible a replacement using the same inverter I ended up ordering another set of the same Vision 100Ah batteries as a drop in replacement. We’re looking at buying a house relatively soon and then get a proper inverter + LiFePO4, so we just need something cheap to last ~1-2 years as a power out solution until that happens.
After chatting with @JacoDeJongh a bit he also recommended monitoring the batteries better and mentioned the BMV-700 that was for sale here. As it happened to be 5 minutes away I went and picked it up the next day. Now we’re just waiting on the batteries. At least we can watch TV for ~45 minutes if there’s an outage…
My questions though:
What size cable should I be using here?
- As the batteries will connect to the shunt first I’ll need more cable and if what I have at the moment isn’t right I’ll just do everything over.
- Using this calculator it recommends that on 1m I use 10mm² for 60A (1440W / 24V if my math is correct), but 25mm² for 100A (2400VA / 24V). That changes to 10mm² and 25mm² for 2m at 3% loss and 25mm² and 35mm² for 2m at 1% loss. I’m not exactly sure which one to use.
Will I need DC fuses?
Anything else to think of?
Just some background again:
This is a Mecer IVR-2400LBKS 24V inverter that I bought as a trolley system, so it simply plugs into the wall and has 3 AC out plugs.
The capacity shows as 2400VA / 1440W hence the above calculations.
It didn’t come with any fuses, so there’s 2 cables from each end running into the inverter and the cable to put them in serie.
Thanks again for all the help as always.