... Since we agree that avionics can do either voltage,
Most modern avionics, yes.
... lights, alternators, battery, and starter, no problem;
Well...
Lights: 24V lights available, pricey specialty aircraft item. Inexpensive automotive lights are typically 12V only.
Alternator: 24V alternators available, but not standard in the kit, less common. Pricey specialty aircraft item.
Battery: 24V batteries available, but not standard in the kit, less common. Pricey specialty aircraft item. Also typically bigger and heavier than 12V Odyssey batteries typically used in RV's.
Starter: Yes, 24V available, likely no difference from 12V, or configurable for either.
so what's left? Servos? It all come downs to just servos?
Ray Allen trim servos: 12V only.
Van's flap actuator motor: 12V only.
Fuel pump: Van's standard fuel pumps are 12V only. Equivalent 24V pumps may be available from other sources. Don't know.
Master and starter solenoids: Van's standard solenoids are 12V only. 24V solenoids available from other sources, but less common, and typically more expensive.
Heated pitot: Brands commonly used in RV's are 12V only. 24V pitot tubes available from more limited sources that are typically more expensive.
Just a few off the top of my head. There may be others.
The number of items unavailable 28v shrinks daily, ...
That may be, but 12V is still far more common, and 12V items are not disappearing. Most modern avionics will continue to support both into the foreseeable future. And anything with roots in the automotive world is still also firmly planted in 12V.
... and tso'd converters are readily available.
Another box. More $$$, more complexity, another failure point. Run a common 12V system and you'll have no need for a converter.
And consider this; just as those with 28v eventually want something 12v, those with 12v might find some day they want some gadget that needs, or prefers, 28v ...
Yes, hypothetically that could happen. But in reality, it just doesn't very often. Most of the fleet is 12V, and you will hardly ever find one with a converter to 24V. It's still a 12V world, and will likely remain so for the foreseeable future.
... could I modify the op to "12v or 24v yet?"
The "yet" might be a bad assumption.
An RV could certainly be built with a 24V electrical system if you're determined to do so. But it would entail significant extra work and cost associated with deviating from standard and commonly used parts, and ultimately all this is for dubious benefit that's marginal at best.
I don't think you're going to see many RV's with 24V systems any time soon, simply because, again, the extra work and cost is significant, and for most of the fleet there just isn't a very compelling reason to do it.