Would suggest that the present alternator can cope with the increased battery capacity, it just takes some more time to bring them up to charge.

When charging begins the alternator at first works hard at full output for some time, but as soon the batteries becomes partly charged their internal resistance grows and reduces the charging current irrespective or alternator size.

It is only during this first part of the charging that alternator output has significance, and you could easily check yourself how long the period of full charging Amps is. This will give a hint on how much a bigger alternator may affect the time.

When continuing charging the current tapers off more and more, and reaching full charge by running the engine would take such a long time that it is not feasible to attempt this. Instead shore powered charging overnight is needed now and then to bring the batteries to full charge, and keep them in good shape.

The recommendation is to begin charging when the batteries are 50% discharged, to keep track of this a battery capacity meter is required.

Another very important aspect is the cable size and proper connections.

If the full charging current is less than the nominal alternator Amperage it would be advisable to check the size of the cables to the batteries, and possibly replace them with bigger ones.

Referring to charging voltages get the battery supplier's recommendations, and compare these with the voltages measured at the battery (not at the alternator).

Kind regards

Lars