The 13.5 will be fine at night. During the day probably not enough. You can follow all suggestions about isolating the cab, windows etc. and see how you like it.
I have the 15K BTU on a 24' no slide. It is adequate but not much beyond that. Personally I think if you find the 13.5 inadequate, I doubt exchanging it for a 15K would be that much better. If the 13.5 is inadequate, I'd add a second 13.5. You may be able to wire in a second 20 amp service just for that AC. With two 13.5s running you'll be cool as can be. At night run the most distant one from your bed.