Your MBR has ducting from the 15K unit in the form of a AC vent or is the MBR wired for a second AC? Which I doubt with it being a 30amp Rig.
The ducting will be the least of your problem. If your MBR is not wired for a second AC you will have to get wire from the breaker panel and install a second AC breaker. If you could do that I would go with a small AC like a Dometic 7k btu that would go in a Truck Camper. You still would not probably be able to run both AC's at the same time. But you could put in a A/B type switch so you could run one or the other.
If you just want more cooling when sleeping you could try shutting all the AC vents except the bedroom when sleeping?