For the breakaway brake system, I'd probably just connect it to one half of one of the battery pairs for general use in as much as the only time it should operate is when there's an emergency, and under those circumstances an uneven load on the batteries is the least of one's concerns.
The simplest and probably best overall method of getting 12V from your 24V battery bank is with the use of a "battery equalizer". With this, you connect your intermiedate 12V leads on all the batteries to the equalizer, and take the 12V from this lead. The equalizer has circuitry that basically forces the two 12V sides to have an equal votage at all times by "moving" current/charge from one to the other. (Technically, I think it's closer to a voltage-controlled buck/boost converter where the output, at the center voltage point, is always one-half the total input voltage.) There are a few nice things about such equalizers: first, the equalizer only needs to supply half the 12V current, the other half coming from the "lower" half of the battery bank directly. Second, instantaneousl current demands in excess of the equalizer's capacity can be readily accommodated, in as much as it will work to pull the batteries back into equal charge after one is depleted more than the other. Third, it's generally possible to use a 12V charger with them to charge the whole 24V battery bank.
