I disagree, just because some coaches are built with parallel batteries it is still bad practice (done for a price point). Yes it can be done, yes match new battery with new battery.
There are three parts to this, drawing power, charging and idle state. I have no issue while drawing power, the batteries can be mismatched and still operate safely as the power will be divided according to available charge.
The problem occurs during charging, yes each battery will absorb its own charge according to internal resistance of the chemical state of the cells. At some point the battery will reach its charged state partially reflected by the voltage. The other battery may not be at the same state. This sets the stage for either a overcharged or undercharged battery. An overcharged battery can be damaged and can get hot and cause it to lose water more so than the other battery. An undercharged battery is wasted.
At idle the batteries charge and discharge each other looking for middle ground, still you have a loss efficiency.
BTW the word "power" is usually referred in electrical circuits, "energy" covers everything. I could have used watts or work to be more specific.
Even batteries in series are not perfect because any cell can be different.
But when given a choice go with batteries in series is safer more efficient and may last longer.