I reached out to Dometic, who is one of our prominent AC unit manufacturers, and he said that whoever you heard that from doesn't know what they are talking about and it is perfectly acceptable to use ducted and non-ducted units in an RV.
For your application I recommend the Atwood Air Command Rooftop RV Air Conditioner # AT15025-21 because it is a non-ducted unit that has a lower Btu rating of 13,500 which is going to be great since you are mainly using this for your bedroom. A great... view full answer...
We do have outside shower boxes like what you are looking for like the part # PF67FR which is 11 inch by 6 inch or we have the # PF247201 which is 5-1/2 inch square. view full answer...