Your otherwise excellent article has a couple of important omissions. The schematic diagram shows two deep-cycle storage batteries connected in parallel, but neglects isolation diodes and fusible links, and there is no mention of either one in the text.
An isolation diode prevents one battery from discharging the other, while still allowing both batteries to be charged from a single +12VDC connection. In actuality, as long as all batteries in a storage bank are diode-isolated, any number of batteries may be connected safely.
In a worst-case situation, and if directly connected to another, one battery (for whatever reason) could present itself as a dead short to the remaining battery(ies), draining all the ‘juice’ quickly enough to start a nasty fire and/or explosion.
I would also install a fusible link at each battery’s positive terminal, just in case… it would act as a high-current fuse, preventing the sort of massive current drain that could really heat things up in a heartbeat.
In what I would consider a safe setup, each battery is contained within its own plastic battery box (vented to prevent the buildup of hydrogen gas) to catch acid leakage, connected to its respective fusible link, to a high-current conductor ‘manifold’ (I have used flattened copper pipe with holes drilled for connector bolt/nut/washer assemblies), each battery with its own isolation diode connected in series with its fusible link.
If a catastrophic failure were to occur, the fusible link would blow, removing the defective battery from the circuit altogether.
Talk to an RV dealer for more details, but the practice of directly connecting two or more high-energy storage components, without fusible links and isolation diodes is a potentially dangerous one. I’m surprised the schematic diagram didn’t catch fire on its own.