Hi @rturrisi,
The reference implementation for all autopilot-related functionalities is the Navigator support in ArduPilot’s hardware abstraction layer, since that’s the only autopilot firmware we’ve done appreciable development for and testing with. This Pull Request shows the commits that were used to add SBUS / RC Input support for Navigator, which is hopefully a helpful starting point.
I believe it’s not enabled by default, and RC input is not guaranteed to work at all in ArduSub. If you want to try it out, the recommended (/verified working) approach is to: