Hello Dwafer,
Based on the principle that for a current to flow in a conductor, there has to be a potential difference, then regardless of battery type or capacity, if two batteries have the same terminal voltage and they are connected in parallel, then no current will flow between them. So that fine. The problem arises if the batteries discharge or charge profiles differ ( for example even two batteries of the same make and model, but of different ages or state of charge) there can be some very high charge balancing currents created. Its these sorts of event that could overload cables and cause damage. Its generally accepted that allowing such currents to arise is a bad idea.
The thread was exploring new battery constructions which blurred the differences between cranking and leisure batteries. If the manufacturers claims are correct then another option for caravanners is to opt for one of these new technologies.