If you have a Harbor Freight in your area, and get junkmail flyers, they often have "free with any purchase" coupons for their cheap DMM, which is good enough for most troubleshooting. (as long as it actually works in the first place; some don't). So you can buy some <$1 item and get the meter free with it.
Unfortunatley you can't safely use the charger on just one battery, as it likely isn't designed for that kind of voltage range.
But you could parallel connect all the batteries together, + to + and - to -, and then they will all equalize their charges. Keep in mind if there is a large imbalance there can be a large current flow and wires can get warm or even hot. (but if there is that much imbalance, the lower-voltage battery is probably damaged anyway, as it means it's sat for a long time at a low voltage, so it's sulfated inside).