Lads, I have finally been successful of running Xplane in bootcamp on my MBP with an eGPU. After a lot of frustrating trial and error, it finally worked. My current system consists of a 2018 MBP with the Radeon RX 555 graphics card, and a Radeon RX 580 8GB graphics card in a Sonnet 350 box and three 24 inch monitors. I now get around 25 FPS with medium setting in Xplane through Bootcamp which fulfills my needs. I had attempted to run Xplane with the eGPU in MacOS but as some of you already know, this doesn't work. After confirming that the Bootcamp process can work, I would consider a Vega 56 or 64 card in a larger Sonnet box. The Sonnet 350 doesn't offer enough power to charge the MBP through the thunderbolt 3 cable.
In case some of you are trying unsuccessfully or planning to try, I'll outline the process that I used that was eventually successful. I will warn you that it took the better part of three days and multiple threats of divorce from my wife to get it all sorted.
Initially, I was following the process stated at bootcampdrivers.com but I had to deviate from their process and work from scratch because of the problems with installing and running Xplane. I started by installing Windows 10 through bootcamp, followed by the Windows initial setup. Then installing bootcamp in Windows (it is self prompted to install) followed by a restart. I then deleted the iGPU driver in safe mode using a DDU program and restarted. I then connected the eGPU and downloaded and installed the drivers from the Radeon website. After installing the eGPU drivers, the external monitors had to be used through the eGPU as the MBP screen remains black. Then I had to reinstall the iGPU driver (Radeon 555) and enable it to be able to download and run Xplane. This problem with not being able to download or run Xplane without reinstalling the iGPU driver may be unique to my system as I couldn't find anyone else with this problem. It should also be noted that I can only run Xplane on my external monitors and both the iGPU and eGPU drivers are installed and both are enabled. The AMD software that was on Windows for adjusting the GPU settings has somehow disappeared and I'm too scared to look into that issue at this point.
Good luck to you all and please let me know if you have any suggestions.
R.