Let's start by pointing out that you are correct.
There is a lot of contradictory advice out there, and this is probably about the best place to ask. The trouble is few people actually understand what they re writing about, and are just repeating what they've been told. Some are trying to justify their choices.
You can skip this next paragraph unless you really want to know:
I think the 1+1 = 2.7 is referring to useable capacity. If you need (say) 60Ah (that's the equivalent of 1 amp for 60 hours or 2 amps for 30 hours, but it is not the equivalent of 60 amps for 1 hour - if you take high currents, the
battery capacity is MUCH smaller. Leisure batteries are rated on the assumption that you take the power over 20 hours or longer. It gets worse: Normally you can only charge a
battery up to about 90% full in a motorhome. You should not discharge it below 50% full, so the useable capacity is about 40% of its official rating. So if you did want 60Ah of power from a
battery, it needs to be a 150Ah
battery bank, at least.
You probably do need to replace the batteries, but best to fix any suspected charging problems first. Running a
battery down even once will damage it. Do it a half dozen times and you need to start looking for a replacement. So use the old, probably buggered, batteries as your guinea pigs and only fit new batteries when you know you have a non-hostile charging environment.
First thing to do is buy a multimeter. You can get them very cheaply, but it is probably best getting one that costs between £5 and £10.
While you are waiting for it to arrive, take the batteries out and charge them in the house (or on the charger in the van if you have hookup where it is parked) for at least 24 hours to get them fairly full. Put the batteries back into the motorhome and measure the volltage across the
battery terminals. See if it is what the panel display says.
Start the engine and see if the voltage goes up - it should, both on the panel and on the multimeter. That tells whether the charging circuit is working at all.
Next step: Leave the batteries with nothing connected (disconnect one terminal on each
battery to be sure) for one hour and measure the voltage across each
battery's teminals (separately). Then measure the same thing 24 hours later.
Post the voltages here so we can see the position: it should show whether the batteries are self-discharging or not.
