1989 M998 - Cranked but would not start

I am a new humvee owner here. I got the M998 for a few weeks and started and ran great.

last weekend, I thought I took the M998 out for a casual ride to the coffee shop, but it wouldn't started. The wait light did come up for a couple seconds. So it seemed the glow plug controller was in working order (I assume).

The battery gauge was LOW (on the RED, left side). I used a volt meter to check the two batteries and it was 22.xx volt. Should this have been 24V or higher? was this the reason the M998 wouldnt start?

Any advice to a new owner would be greatly appreciated.

That's kinda low. Did it attempt to start at all? Starter turn?

I just went out and tried again and it seemed like the battery issue. It's much weaker than last week. The starter turned.

my next question is that could I use the CTEK-US-Multi-7002 battery tender (link) to charge the batteries? if so, do I separate the two batteries or it is not necessary to separate the battery.

Charge the batteries independently as 12v with a normal battery charger.

Second that simp. Get them boosted and then try again. Use a voltmeter or multimeter to check your charge coming in to them while running. Don't trust the stock gauge.

just went home for quick lunch and hooked up the multi-meter to the battery terminals, when I turned the switch to the wait light, the multi-meter showed it dropped down to 16V. I can safely conclude it is battery lost their charges issue. Will charge them this evening when get home later