Im about 99% sure it only flushes the front tank.
I looked the your model before we bought our sabre 36qbok. (Almost identical floor plans and trailers). We even tried getting the factory to install a rear black tank flush when we ordered, but they wouldn't. I guess there is a code, and they don't have room for a vacuum breaker, or something like that. We ended up having the dealer do it before delivery since the factory wouldn't. Its nice, but the hose included in the kit has given me problems with leaks at the crimps.