If there are just two 12 v terminals you can only get 12 v.
If there are two equal 12 v windings (two pairs of terminals each providing 12 v) you can connect then in series to get 24 v. It is important which way round the two windings are connected because one way you get 24 v, the other way you get nothing.
Use a voltage devider or a transformer.
Step down transformer.
Change the trailer light bulbs to 24 volts and supply a 24 volt source to power them, in that order.
European circuits need a transformer. North American circuits have 120/240 Volt supply. Using a neutral and one of the hot lines would work. To understand, think of two twelve volt batteries connected together in series. The voltage on the outside two terminals is 24 volts. The voltage from any outside terminal to the center is 12 Volts. With two wires from each of the outside terminals and one from the center, giving you the option of 24 Volt or 12 volt with three wires.
All you have to do is change the bulbs to 12 volts and supply the trailer with a 12 volt power source. If you leave the original 24 volt bulb in the trailer they will only glow at 1/2 of their rated wattage.
Usually in the air handler
Yes, a 12 volt battery will be damaged if charged with a 24 volt charger.
For the 120/24V transformer, the primary side is around 50ohm, the secondary side is 2~
you only need two 12 volt batteries to make 24 volt. connect them in series
With a Step Down transformer. But you will probably have a hard time finding a 240 volt unit. 120 volts units are readily available. <<>> It depends on how big of a transformer that you are looking for. There are many control transformers in the 50 to 500 VA range that have dual primary and dual secondary. The primary side can be connected to 240 volt or 120 volt and the secondary can be connected for either 12 or 24 volts.
12 volt standard, 24 volt optional.
use two 12 volt batteries in series.