... The reason to "not" do this is that 24 volt STUFF is more expensive than 12 volt stuff that does the same thing.
John, I actually disagree with this and believe that it is a widely held myth.
It's easy to see where this comes from, because, for example, a 10-watt halogen bulb in 12 volts costs less than half what the same bulb costs in 24 volts. However, the big ticket items are really the other way around.
For example, if you want a 2,000-watt inverter, there is no price difference whether you buy that in 12- or 24-volts. However, the 12-volt model will cost a lot more to hook up -- figure to spend twice as much for the DC cables and crimps, and 50% more for the fuses and disconnect switches.
OTOH, if you want a 4,000 watt inverter, you will save a ton by going with 24 volts -- there are no 4,000-watt inverters in 12-volt, and so you need to buy two 2,000 watt models, for 50%-70% more, plus all the duplication of cables, crimps, and extra terminals.
Fuses and circuit breakers for your DC loads are either the same price or less money for 24-volt than 12. That's because below 30 volts, it's all the same stuff (there is no difference between a "12-volt" breaker and a "24-volt" breaker), but you will use lower values for 24-volt loads than 12-volt, and when there is a price difference in these items, it is always the higher current items that are more expensive than lower current items.
Sensor VSD pumps and Danfoss compressors are dual voltage -- they run on 12 or 24. Webasto heaters are the same price in either voltage.
You will also save a ton of money (and weight) on battery cables, inter-ties, solenoids, and DC load wiring by using 24 volts instead of 12. And, of course, if you have a 24-volt alternator, charging a 12-volt house system from the main engine will cost a bundle.
All of this far outstrips the extra cost of 24-volt bulbs (the fixtures are the same -- no such thing as a "12-volt" incandescent fixture vs. a 24-volt one) and even the cost of a 30-amp equalizer.
Lastly, I will point out that 120-volt items are almost universally less expensive than low-voltage DC items to do the same thing, and that includes lights, pumps, and even refrigerators. So it can be argued that, to save money on individual components, you should use more 120-VAC stuff, and get a bigger inverter, which, as I said, is cheaper to do in 24 volts. (The downside to this, of course, is that you pay a loss penalty through the inverter, so this is not the best strategy for extended boondocking)
And, 12 volt stuff is readily available EVERYWHERE.
Well, OK, it's easier to find 12-volt bulbs and pumps. But how hard is it to carry a spare bulb or two?
The vote has been cast by the RV and bus converter industry. They build 12 volt house systems, as far as I know.
Every major bus converter uses 24-volt house systems when starting with a 24-volt bus chassis. They use 24-volt inverters and 24-volt distribution panels, and convert down to 12-volts with a battery equalizer for those things that simply don't come in 24-volt, like FanTastics.
The rest of the "RV industry" works on 12 volts because that's the alternator voltage provided to them by the chassis manufacturers. If you start out with a Ford F450 it makes just as little sense to put in a 24-volt house system as it does to put a 12-volt system into an MCI or Prevost. Less, actually, because stepping the 12-volts up to 24 to get any alternator-driven battery charging is much harder than tapping a 24-volt system to get 12 volts where needed.
Using a Vanner for 12 volts is a problem in that the Vanner usually can't handle the full load of the house cause a beefy Vanner like that is expensive and not efficient.
That's not how an equalizer works. The equalizer does not need to be able to handle the full load -- the batteries do that. The equalizer then makes it up over time. This is why you draw the load off the center tap and the equalizer is also connected there. It's possible to use a Vanner as a straight voltage converter (no center tap connection -- only the loads are connected to the output), in which case you do need one sized for the full load -- but then it's not really an "equalizer." You size an equalizer based on the time average of the 12-volt loads, usually only a small fraction of the total, and not the maximum possible 12-volt load.
Sean designed his system from the ground up and I think he has a Vanner to make 12 volts for only those systems he can't get in 24 volts. Now, that gives me paws cause he is the kind of knowledgeable guy that could "build" stuff that would run on 24 that used to use 12.
Not much knowledge required, for what I did. I changed all the incandescent bulbs to 24-volt items I bought on the Internet. For the handful of places I wanted to use LEDs, I simply wired them together in series pairs. I sold my 12-volt Webasto on eBay and bought a 24-volt one for the same price. And, of course, I bought a 24-volt inverter, two 24-volt water pumps, and a 24-volt fridge.
Now for the best part -- all I needed to get my huge 24-volt alternator to charge the house batteries was a $100 relay:http://www.busconversions.com/bbs/index.php?topic=12028.msg126976#msg126976
YMMV, as they say...