Well, that depends.. are you looking to make a 12v circuit or a 36 volt circuit? Presumably, these batteries will be for powering the accessories in the camper compartment, rather than the cab compartment. Honestly, without a wiring diagram available and without knowing what kind of circuit you're trying to wire up, you're better off to go to an RV service shop.
Connect two 12 volt batteries in series and you will have 24 volts.
No.
Actually, if you connect the above described LED to 220 V, it will immediately burn out then the circuit will be "open" and the resistance will be "infinite".
Sure you can and you will end up with 36 volts. Connect them in Parallel and you will have 12 volts.
Yes.
Connect them in series and you will have 24 volts but you will have the same A/H as one of the batteries. Volts double but A/H stay the same.
Connect them in Series and you will get 24 volts but the amperage will be the same as 1 battery.
pass 240 volts through it
Connect three 12 volt batteries in series. You will triple the amperage but the volts will remain at 12. Click the link
it is possible to .connect two led in reverse polarity. and then one end connect with a 100k resistance then it can connect directly to230 volts
Click the link for a diagram.
You will have a 12 Volt source with double the charge capacity.