As far as truck campers are concerned, this forum here on RV.NET seems too be the most useful source of information. It has the most participants and posts
New posts on the NATCOA site were few and far between. The same can be said for the iRV2 and RVForum sites. They HAVE truck camper forums, but not a lot of activity on them. Sometimes a week or more between new posts.
The Escapees forum doesn't even HAVE a truck camper forum.
The Truck Camper Adventure site has an associated forum. But it's not as busy as here.
Wander The West has a pretty good forum devoted to truck campers.
Finally, the Ford Truck Enthusiasts forum has a sub-forum devoted to "slide in campers" that I find useful. I don't know if any GM or Ram truck forums have anything or not.