How Do I Charge, and Maintain 2 New 6 Volt Batteries???

Here is what I have to use for a charger, voltmeter, etc.

Schumacher SE-5212A Automatic Handheld Battery Charger Amazon Link For Details




My on-board charger ... these are pics I took of it in my trailer since I can;t find the exact charger online for reference.





My voltmeter that I just bought ... will this work to keep track of the voltage in my batteries?
Equus 3320 Auto-Ranging Digital Multimeter Amazon Link For Details




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I'm thinking of getting one of these and mounting it on the battery box to just see at a glance without using multimeter
LCD Digital Volt Voltage Panel Meter Voltmeter 7.5V-20V Amazon Link For Details




Or maybe one of these and just hook into my cigarette lighter in the bedroom ... anyone tried this kind of thing?
Equus 3721 Battery and Charging System Monitor Amazon Link For Details




Would love your thought and feedback on all of this ....

Is that Shumacker charger good to charge 6V batteries? If so what do I put it on when charging?

How good is that on-board charger that you are aware of? (plan on getting PD charger but not till maybe next year).

Is that Voltmeter a good one to test the actual 12v level in the batteries? If so I just put it on the 12v Batter Load Test setting to view the current level?

Anyone use these other small add-on volt meters for quick glance at the voltage level? Do they work well?

Thanks for all the information you can provide. :)