You will need to purchase a 24 volt DC to 12 volt DC step converter; however, a 24 volt battery bank will allow you to use a smaller charge controller or just one charge controller instead of two, so that will offset the cost of the converter. Probably would be cheaper if you are ideally using an expensive, high end controller. (Charge controllers can handle only so many amps. If you had a charge controller that could handle 400 watts of solar with a 12 volt battery bank, it could probably handle 800 watts of solar with a 24 volt battery bank.)
If you had high demand AC items like a 240 volt, AC clothes dryer, then higher voltage battery banks make more sense because you can get higher wattage inverters to run those types of items. A 12 volt battery bank won't run an inverter large enough to power a clothes dryer like that. Well, maybe they do make one, but the losses would be high and the cables and fuses from the battery would be MASSIVE $$$$.
A deal breaker for me is that converters have fans. Maybe some don't, but a lot do. Most charge controllers don't use fans but use heat sinks instead. I like it quiet.
The most critical part of energy usage occurs at night or when the sun isn't out like on cloudy days. A converter has losses, so when using a 12 volt appliance connected through a converter on a 24 volt battery bank will result in the appliance not running as long. Using a 12 volt appliance directly connected to a 12 volt battery bank will result in the appliance running longer because there is no loss from conversion; the wires leading out from the converter will be the same gauge as the wires leading out from 12 volt battery bank. The only place you will reduce voltage drop is from the battery to the converter and that will extremely likely be a very short distance. I would rather spend the money on 6 feet of 4 AWG instead of saving money by buying 6 feet of less expensive 6 AWG.
Since the distances from my charge controllers to my battery bank are relatively short, I opted to spend the extra money on larger wiring to reduce losses instead of spending the money on a noisy converter to reduce losses. (Wire isn't that expensive.) Because that's really the only advantage of having a 24 volt battery bank- to mitigate energy lost through wires and inverter losses. There's more than one way to reduce voltage drop.
While I spent more money on two charge controllers to handle all of my solar, it is a more efficient and effective way to capture the energy. That also translates into more energy being pushed into the batteries. That is the end result you want, right? More energy?
You can get one charge controller that is less efficient to save a tiny amount of energy from reduced voltage drop using a 24 volt battery bank.
Two charge controllers provides redundancy. If one goes out, half of power is still being produced. Half is better than nothing. If a system has one charge controller and it goes out, zero energy is being produced and there is still a DC to DC converter that will eventually fail as it won't last forever. A system with no converter will have nothing to go out to and still provide 12 volt power; there is nothing to fail. With everything else being equal that system is 100% reliable compared to a system with a converter. I place a high value on that, but that may not be as big of a deal for some people.
With long wire runs a 24 volt battery bank starts becoming appealing. Wire is expensive. But I doubt you have an 80 foot run of wire from your charge controllers to your batteries. If you are like most people on here, you will have 8 feet max with probably less than 6 feet. (The runs of wire are the length of the negative wire plus the length of the positive wire which could be as long as 80 feet in a 40 foot RV. This also assumes that the charge controller is placed as far away from the batteries as possible. Charge controllers need to be as near to the battery bank as possible.) The only place you will be saving wire is from the converter to the batteries. All other wires will be the same gauge and almost exactly the same length. Unless you are running 24 volt appliances (harder to find and more expensive). The decreased voltage drop going from 12 volt LED lights to 24 volt LED lights is minuscule since the amps are so small anyway.
That is the long explanation. Simply put, for your small demands and short runs of wire a 12 volt system is ideal. You could do a 24 volt system, but the gains you would get would be very minor. Totally not worth it when there are other ways to mitigate voltage drop and/or increase the amount of energy going to the batteries.
