What size Air Con for Bedroom???

They told me when I picked her up last month that I'm pre-wired for a second. I was thinking the 13k btu

13K btu is the norm. They usually install a low profile unit up front because it is the highest part of the 5er. Cost varies due to several factors; factory prewired, stand alone unit or connected to existing system, dual or single zone thermostat, and other factors.

The biggest that will fit the hole. You can turn one or both on low if need be so they can run longer. or just run one until you need two.

I have 3 on my 41ft 5W the 2 that came with it> 15k BTU (MAIN),and a 13.5k BTU (BEDROOM) both dometic briskair duotherm's. I needed a 3rd for my rear LR So I installed my self a 13.5k BTU Dometic briskair duotherm 2.
Now everything works out well.
FOR YOU geta 13.5k btu maybe the Penguin II Heat Pump will be the best bet and it doubles as a electric heater.

5000 btu ought to be more than enough. Are you sure you need that additional a/c capacity? Sometimes you can have too much and the air gets colder without getting drier.