Does your rig have a ducted system that the rear bedroom AC ties into, if so a 15K unit might be more advantageous than the 13K unit as it could assist the living area AC on hotter days. Without ducting a 13K unit along with the dump feature should cool down most RV bedroom areas quite nicely. We don't ever turn our AC units off on very hot days, don't want them to have to remove heat from rig all over again
We have large opposing slides and lots of windows in our main living area, often times the rear AC is set to deliver it's frigid air into the ducting system to support main AC on those blistering days of the year. Survived 110+ degree weather in TX & AZ on dozens of occasions, our AC units endure heavy usage and are also utilized in fan only mode to provide white noise for sleeping.