Coleman Mach 15 A/C & Generac NP50-G Genset

RV Gensets that are 5kw and larger will run 2 roof AC units with no problem. RV Gensets for YEARS have had a 30 and a 20 amp breaker with the 20 amp dedicated to the 2nd AC. So, your 20 amp breaker on the Genset is fine for running your new AC. The SPEC for your AC is a 20 amp breaker. It seems that your unit may be short cycling? How long does it wait before trying the 2nd compressor start? If less than 10 minutes, that may be your problem. If not, then YES, a Hard Start kit will definitely help you out. Coleman makes a Hard Start kit or you can purchase one locally. The NEW 15k will pull 14 amps at 95 degrees and the momentary surge amp on a 20 amp breaker will not cause the breaker to trip. If it does, the odds are the Genset Governor or running RPM's are not to spec. It is also possible you have a defective new AC unit(compressor) that does not release the load(equalizing) when shutting down. Can you test the new AC on shore power? Doug