A twelve volt charger will charge twelve volt batteries only unless it has a feature that allows you to change the chargers voltage. But on fast or slow charge 12 volts is 12 volts so most 12 volt chargers are only good for 12 volt batteries.
Yes you sure can.
amps
Twelve volt batteries can be used for a number of different things. For example they can power small electronic devices. There are also a number of different brand such as Energizer and Deltran.
Assuming the truck has a 12 volt system, connect the batteries in parallel, that is positive to positive and negative to negative.
A group of ten (10) batteries composed of 6 Volt and 12 Volt types will have six (6) each, 6 Volt and four (4) each 12 Volt batteries. The contributing voltages will be 6x6 and 12x4 for a total of 84 Volts.
The charger needs to be set to charge for the voltage that the battery is designed to produce. Most car batteries are 12 volt batteries.
There are 6 volt car batteries, but you are only likely to encounter them in really old cars, like late 50s early 60s.
All auto batteries are 12 volt except very old cars prior to the late 50s.
I would opt for the two twelve volt batteries in parallel. This configuration will give you the voltage that you need for the system to properly operate plus the endurance time that you can draw from the batteries will be doubled. The amp/hours capacity of batteries when in parallel are additive.
A 7.2 volt battery is used in most remote control cars. They are also called RC cars and some places sell RC batteries for these specific cars. There are most likely other batteries that can also be used.
connect the 2 six-volt batteries together one terminal each positive to negitive, this will make a 12-volt combination, then you can connect the jumper cables, one side to the 12-volt battery and one side to the two 6-volt terminals, + & -