Roof mounted RV air conditioners are noisy when using high fan setting and less noisy on low fan setting. In a 19 foot trailer you should be able to run it on low fan to stay cool. It will still be noisy. Not much can be done.
One of our air conditionders had a high pitched buzzing sound. It turned out to be two pieces of tubing that were too close together and vibration caused the buzz. I put a piece of foam between them to stop the buzz.
I would equate the low fan RV A/C noise to a hair blow drier on low speed around 15 feet away.
By the way there are two noises... the fan which is always on and a deeper sound when the compressor kicks on. The compressor cycles so the sound comes and goes.
We normally watch TV with the sound at 20, when the A/C is on I turn it up to 35 to be able to hear what the actors are saying. The air inlet for the A/C unit is in the ceiling above where I sit to watch TV (dumb design).