New electrical service to detached ADU/garage
Normally, you wouldn't be able to do as you describe

Normally, a carriage-house ADU setup (with the ADU atop a garage space) is set up on the only garage on the property, with use of the (detached from the main house) garage shared between the main unit and the ADU. Were that the case in your situation, your proposal would be no good, even though your utility's metering rules are silent about separate metering vs. master metering, as it would involve serving what effectively can be treated as a common area of a two-family or multifamily dwelling (the garage, shared between both/all units on the property) from equipment that serves an individual dwelling unit (the ADU), which violates the intent of NEC 210.25(B), if not the letter:


  (B) Common Area Branch Circuits. Branch circuits installed
  for the purpose of lighting, central alarm, signal, communications, or other purposes for public or common areas of a two-family dwelling, a multifamily dwelling, or a multi-occupancy
  building shall not be supplied from equipment that supplies an
  individual dwelling unit or tenant space.


Since that's not your situation, though...

Thankfully, in your case, the main house has its own attached garage, separate from the ADU's garage, which eases things significantly, as it allows us to treat this in a fashion much closer to what you describe, which is basically a detached duplex configuration, with each dwelling-unit/garage combination being served and metered separately, but with the garages treated as part of their respective dwelling units.

This makes your existing plans for the ADU much more feasible.  Load-wise, your ADU comes out to a conservative 102A of load using the Article 220 procedures, given:


800 sf of ADU at 3VA/ft2
3 20A small appliance branch circuits (2 kitchen, 1 laundry)
an 8kVA range allowance (good for ranges up to 12kVA)
a 5kVA allowance for an electric dryer
a 4.5kVA allowance for an electric hot water heater
and a 1.5ton, 13 SEER air conditioner outdoor unit (9A @ 230VAC).


With this, we add 3600VA for the currently proposed RV receptacle and another ~800VA (2400VA at 35%) for lighting in the garage, which puts us just shy of 125A, well within what your 200A service can handle.

Equipment-wise, I would use a 200A meter-main on the outside of the ADU/garage that is configured with a single main disconnect feeding a small (8 spaces is common) panel.  This provides enough room for a separate shutoff for the ADU feeder by itself, as well as shutoffs for the garage and RV-bay feeders, while allowing a 200A feeder to the ADU, if so desired.  Since your utility's metering rules are silent on whether to use ring-style vs. ringless metering hardware, and the approved meter socket list specifies both ring-style and ringless sockets, I would speak with the utility for details.

If you insist on separate metering of the RV bay, then we can use a WEP2212EV with the 200A portion feeding a tap (in a 2" RMC nipple) to a 125A panel for the garage as well as providing the main 200A feeder to the ADU, and the 50A portion providing a separate feed to a RV-panel box in the RV bay.

As to the subpanels: GO BIG OR GO HOME

The key number with panels more often than not is not how many amps a panel can handle (a 125A panel is more than adequate for anything you're doing in the garage, for instance), but how many spaces the panel has.  Considering that putting a large panel in now is far cheaper than upsizing the panel later, I would use a 200A, 40- or 42-space panel for the ADU and a 125A, 24- or 30-space panel for the garage.  Both of these can be main lug or main breaker panels at your leisure (unless the garage is on a feeder tap; in that case, the garage panel must be main breaker in order to protect the feeder tap and busbars properly), since they are being fed with feeders from the meter-main/meter-pack hardware outside, and can be of whatever current make and model you wish.

Your plan to run a fat PVC conduit for the feeder to the ADU is wise; however, you'll need to LB it from the wall into the back of the meter-pack/meter-main box, and perhaps use a reducer to narrow it down to 2.5" or even 2" for the nipple connecting the LB to the meter-pack, depending on the size of the knockouts available on the meter-pack/meter-main you are using.  (This isn't an issue from a fill standpoint since nipples &lt;24" long are permitted to use 60% of the available conduit space, vs. 40% for longer runs.)

ADU panel placement

The ADU subpanel needs to be accessible at all times to the ADU occupants, as per NEC 240.24(B)(2):


  (2) Branch-Circuit Overcurrent Devices. Where electric service and electrical maintenance are provided by the building
  management and where these are under continuous building
  management supervision, the branch-circuit overcurrent devices supplying any guest rooms or guest suites without permanent provisions for cooking shall be permitted to be accessible
  only to authorized management personnel.


Given that the garage is effectively being treated as part of the ADU here, putting the panel for the ADU in its attached garage is not an issue; however, there can be merit to having it in a space which is less likely to be blocked by storage or other items, as such blockages violate NEC 110.26(B):


  (B) Clear Spaces. Working space required by this section shall
  not be used for storage. When normally enclosed live parts are
  exposed for inspection or servicing, the working space, if in a
  passageway or general open space, shall be suitably guarded.


This brings us to your plan to locate the panel at a stairway landing; this is acceptable, as 240.24(F) only prohibits locating a panel over steps of a stairway, but does require the landing to be large enough to fit the entire clear floor space required by 110.26(A).

RV provisions

While it's certainly possible to simply have a TT-30 receptacle in a regular junction box wired to a 30A, 1-pole breaker in the meter-main or the garage panel to serve the RV, I would instead run a 50A feeder from the meter-main to a dedicated, surface-mounted RV box.  This provides enough room for upgrading to a 50A RV receptacle later, and just enough room to tuck a 60VA(W) lighting load into the picture, even with the RV receptacle maxed out (we're exploiting the 220.5(B) rounding rule here, but you probably aren't going to be maxing out the RV load in the bay with the lights in the bay on, no?).

That RV box, at the moment, will need to have at minimum a TT-30 receptacle (for your current RV) and a spare space that can accept an appropriately typed 15A AFCI breaker (for the RV bay lights); a Talon TL37US with a Q115AF for the lights will do the trick here, and provide a 20A duplex GFCI for general use within the RV bay as well.