So here are the onboard systems I need to interface with.

From research, the Magnum Energy inverters, Automatic Generator Start, and user panel all use RS-485 to talk to each other and the read-only part of this at least has been decoded.  As I can set the system to start the generator itself (as that is what it's for), tackling this from the read-only perspective should work for now.

This panel looks to be just a couple SPDT switches for the slides and some sort of analog interface for the tank/propane levels.  I should be able to simply wire across the slide switches and tap into the analog signals directly.  I don't know the levels involved yet for the analog.

The single lights controls.  The trim ring snaps off with a pull, but they are screwed in to the paneling with an S2 drive.  I only had a Phillips with me so I'm still not sure if these are SPDT or SPST switches thus far.  There is at least one SPDT switch as the main living area is already 3 way, with one switch at the RV entry and the other in the middle, so I'm hopeful.

Here is the existing centralized AV HDMI over ethernet switch.  I wasn't able to find anyone who has built a library for sending IR to this device yet, and I don't have a remote for it.  AV switching will have to wait a bit.  

All the systems thus far are centrally located and within 2 feet of each other.  I should be able to control all the simple stuff from this one location with a relay board.  The same mirco I task with the analog bits will also be intercepting the RS-485 for the house battery status.

As for the more remote things...

There are also 4 TV's, but three of them are the same brand and so use the same IR codes.  I'll need at least 3 IR blasters. to cover them over 3 isolated channels.  Annoying.

The switch panel in the entryway covers the awnings, more lights, and steps and is far enough away from the other panel that I plan on using an RS-485 masters/slave serial link to it with an Arduino driving a separate relay board.

Lastly I'll need to control the mast rotation and extension.  That's all the way at the back of the RV and so about 10 feet from the main board.  I haven't decided if I want to make that an RS-485 link or just run the wires yet.  I have power there so RS-485 may make sense.