I have three 100ah and one 60ah battery - all are AGM batteries, the latter was installed by the dealer when we picked up the unit.
I've been told that one should not mix batteries with different amp hour ratings. True or False; Why?
Not quite true. Battery banks work best together when the batteries are identical. Two 6 volt batteries need to have the same
amp hour rating and chemistry. They live long and prosper if they are identical. They are connected in series.
You have 3 AGM batteries. If they are 12 volts, the same brand, the same age and the same model line, they will work well together. Even the 60 amp battery will cooperate if it has identical chemistry. 12 volt batteries will be connected in parallel.
If one or more of the batteries have a little different chemistry, you may need to periodically separate the batteries and fully charge each one for 14 to 18 hours to equalize them. Most people would not take the trouble to do this.
The converter in our 2020 Kodiak Ultra Lite (201QB) contains a 3 stage battery charger in the power center. Given that fact that I have installed additional batteries do I need to upgrade the battery charger?
The converter charger may be a WFCO. Mine is a 30 amp model and I use it to charge a 200 amp hour AGM battery bank. The charger will work fine with any size battery bank.
You would probably benefit from a 40 or 50 amp charger. A charger with higher capacity will charge a large battery bank faster during the first few hours. This is more important when using a generator to charge the batteries.
Lead acid batteries absorb high currents when they are deeply discharged. They only absorb low currents when they are finishing the charge. So you could start putting 50 amps in. Current will slowly decrease. After 4 hours current may begin to drop until it is only a few amps.
No matter the size of charger there is still a long slow finishing charge at low currents to do a full clean charge.
Is there any advantage to having a "charge controller" like used for a solar installation?
The WFCO converter/charger is a charge controller. It converts 120 volts AC to 12 volts DC that is used to both run 12 volt appliances and charge the batteries.
A solar array needs a different kind of controller to charge 12 volt batteries. A generator would power the WFCO converter/charger.
I wish you good luck and happy trails ahead!