Like modern cars, many of today's RVs (recreational vehicles) contain a CAN (common area network) bus called RV-C for intra-vehicle communication. RVs are like homes on wheels, but actually contain more subsystems than a typical house. For example, an RV requires holding tanks (unlike an on-grid house) and those benefit from electronic monitoring. Even the most basic RVs have several subsystems, with high-end luxury RVs containing a multitude of them. Unfortunately, it is difficult to integrate those with third party control systems. To solve that problem, Randy Ubillos created an Apple HomeKit to RV-C adapter called RV-Bridge.
HomeKit is Apple's smart home framework, which allows for all kinds of home automation tasks. You can, for instance, purchase a HomeKit-enabled smart thermostat. Once installed, you can then control that thermostat using the Home app on your iOS device. Other HomeKit devices can also interact with that thermostat, like an occupancy sensor that turns off the air conditioning to reduce energy usage when nobody is home. For someone that spends a lot of time in their RV, it would be very convenient to integrate the RV's various systems with HomeKit. This RV-Bridge helps to facilitate that integration.
RV-Bridge is an ESP32-based device that acts as an intermediary between HomeKit and RV-C. While automakers all have their own CAN bus protocols, RV-C is more standardized as most RV builders utilize appliances and systems from the same handful of manufacturers, such as Dometic and Coleman. RV-Bridge fits inside of an RV's wiring panel and connects to its RV-C bus. It then uses the ESP32's WiFi capability to connect to a HomeKit hub device. Several iOS devices are capable of acting as hubs and Ubillos is using an Apple TV for that purpose. The ESP32 runs HomeSpan, which is an open source Arduino library for creating DIY HomeKit devices.
The capability of an RV-Bridge installation will depend on the RV itself and the systems connected to the RV-C bus. In many cases (especially for older RVs), there might be very little (or even nothing) on the RV-C bus. But anything on the RV-C bus will, theoretically, be controllable through the Home app. Control may require that the user decipher the relevant CAN bus messages, but RV-Bridge will let the user reproduce any messages that they like.
At this time, Ubillos has his lights, climate control, and switches integrated with HomeKit through his RV-Bridge. Other RV owners with the requisite technical savvy should be able to do something similar, assuming their vehicle has RV-C.