There have been multiple trike build threads here in the 'non-traditional' builds area. There's really nothing particularly special about 'automotive style' reverse trikes that wouldn't be applicable to discuss here as well, unless you're trying to get into tilting or something like that.
However, if you want RT specific stuff, there used to be a RT Yahoo Group that had a lot of good info, but I'm not sure how to find if any archive exists for it not that the platform has disappeard. Otherwise...
Here's a forum archive:
https://reversetrike.proboards.com/Here's a not very active forum:
https://reversetrikes.freeforums.net/Here's where most of the people probably went:
https://www.facebook.com/groups/953318724689005