The trailers that you mention all have a much lower window to interior volume ratio than the T@Bs. Personally I think that this is a pretty big factor. I think that you need to find some way to cut the amount of sunlight beaming into your trailer. In hot weather I think that most of us use window awnings/covers attached…
As you keep mentioning the problem being related to direct sun, I'd like to ask again: Do you have any kind of awnings or shades over the windows? Or do you have the inside window shades closed (a bit gloomy)? If you look at the ratio of window area to the trailer interior volume, that's a heck of lot of heat pouring into…