Short of replacing the whole thing, there is not much you can do.
To reduce noise from any machine: isolation, separation and insulation.
With the unit mounted on the roof, vibration is transmitted to the trailer shell which acts like a big speaker. there should be rubber cushions between the A/C and roof. And the components inside the A/C should have vibration dampers.
The type of compressor used makes a big difference.
If possible, look for a "Quiet" unit.