Can anyone confirm that this is a reliable method to charge multiple SLA batteries in parallel?
I've seen this scheme widely recommended (from the OP's image source):


I think their calculations are wrong, as each battery has three jumpers between it and the charger connections - some jumpers in the negative lead, and some in the positive, so all should see the same voltage and current.

However, my feeling is that the difference between the various connection schemes is insignificant.  While there is siginificant charging current, there may be a slight voltage (and charging current) difference between batteries with some wiring arrangements, but as the batteries approach full charge, the charging current will reduce, reducing the voltage drops in the wiring - in the end, all batteries will be charged to the same voltage, regardless of the wiring arrangement.

On my boat, I cheat - I have equal-length jumpers from each battery (positive and negative terminals) to common bus bars and current, with the charger and loads connected to the bus bars, so there is no question of which battery has the shorter connections.