Decibel ratings omitted form specifications, for equipment like this sets alarm bells rings. But in practice even if the manufacturers were to quote values, they wold be almost meaningless unless the product is totally silent.
Just consider the appliance manufacturer has no way of knowing how many different wall or roof constructions the product may be used in, and how its mounted will affect how much noise is emitted from the device and its supporting structure, becasue its will work just like musical instrument sound box, and in some installations the small amount of vibration these units inevitably produce will be just right to excite the body into vibrating and thus amplifying it's noise. Some on board water pumps are perfect example of this, they can be heard across a caravan site!
For consistency the measurement of the sound pressure would almost certainly be carried out in an acoustic chamber, which bears no relationship to a caravan, so the measurement wold only allow a comparison between units, and not represent the reality of a real installation.
But I also come back to the effectiveness of AC units in caravans. The power available to drive them is limited and when you look at what its trying to do, and compare it to the AC in a car the it begins to show the difficulty it is facing.
The maximum driving power available in a caravan is about 3kW, and that is expected to deal with a whole caravan which will have a vast surface area and volume which provides heat gain from the outside air and sun, yet the systems fitted in cars are typically rated these days between 10 and 12kW, and yet they have a much smaller surface area and volume to contend with. Even allowing for the slightly better insulation in a caravan, the power to volume of a caravan system is much smaller that that of a car, so its not going to be any where near as effective as the system in your car.