Moving to 24v system, requires step down for house DC?

Not an issue with your batts (Li ?) If you have 60 cell panels instead of 72 there is a problem getting the voltage above 30v for 24v batteries. I ran an experiment with that and it stalled out just below 31v. I had the controller doing 24-24 on Flooded batts.

The pos wire from the breakaway switch to the 12v battery in the trailer is not supposed to have a fuse. Might need a way around that if using a buck converter for the 12v, not clear on the wiring for doing that.

You could make one pair of the 6 batts do 12v, and put the other 4 in series-parallel for 24v on the inverter. You need a separate charger for the 12v battery bank.

2oldman ran a 24v system for a while but changed up to a 48. He used a DC-DC converter for 12v things. Why go 24 at all when you can go 48 would be the question I suppose. Don't know how he ran his breakaway.

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.

I have a Victron unit. Excellent quality. I step down from 48 to 12 at 30a. Works fine for slides and jacks.

When I had 6- 12v batteries I tried cheating and wiring up just one for the coach, but that didn't turn out so well. Throws it off balance with the others.

That's the question. I know there are several (many) users out there with 24v systems, wondering what their solution is other than maintaining a separate battery system for house 12v , which seems to defeat the purpose a bit...

Will 40 amps of 12 volts work the slide motor?

Hadn't considered the brake system, good point, although nominally I would wire into the same stepdown. Don't have a charge line from the vehicle, have ample solar.
Primary concern would be the 'momentary' high discharge from the landing gear and slides

Depending on your 12v needs, you could use either one big step down, or point of load smaller step-downs.

Good quality LED lights should work fine with a 24v system, having built in regulation.

Something else to consider is your emergency brake system, and charge line from the tow vehicle. What's the plan there, separate dedicated battery?