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Either one would do, I'd suggest the trickle charger since it is "enough". How long a period will depend on the amperage it supplies versus what the batteries need. But also, since AGM batteries can't leak acid, you don't need to hide them, you can put them wherever it is convenient.
To check the alternator output, put the plus and minus probes from the meter on the matching battery terminals (an easy and safe place to access) while the engine is running. At "cruising" rpms, you should see 14.4 volts, sometimes as low as 13.8 volts depending on the system and the battery condition. Off by a point or two doesn't matter, there's some float in the meters and tolerance in the systems.