The chance of finding a tech who understands the system you have is almost impossible. When you reported compressor gets warm and little cooling
this points to one of four possibilities, Moisture in system, tech removed to much refrigerant, a bad ground connection causing an interruption of all three phases in compressor or refrigerant flow restriction/blockage.
Knowing more about your system would help. When you purchased boat how well did refrigeration
work? Did compressor get warm before calling tech to look at it? Does your Frigoboat unit have a Speed controller and a red trouble light? Did original owner tell you anything about refrigerator
I always assume when a service
tech tells you to buy a new unit and says pressures are correct he as no idea of what is wrong with your unit.
Frigoboat units do have design faults that end up causing loss of refrigerant and eventually refrigerant flow troubles. Their failures start with leaking Line connector O-ring seaqls. When refrigerant is added small amounts of moisture mix with refrigerant. The operator can easily determine the difference between low on refrigerant and moisture in refrigerant by listening to gurgling refrigerant sound inside evaporator. If sound stops for a while and then starts again and evaporator has cold condensation
on its surface this indicates moisture in refrigerant. If gurgling sound is steady for 15 minutes and no frost system is low on refrigerant.
Bad Ground between control module and battery
that will reduce compressor speed so slow it can not maintain high pressure liquid at refrigerant flow control cap tube. Test to eliminate this ground would run a jumper wire direct from module to battery
eliminating all boat ground wiring
The most serious problem with Frigoboat systems operated at heavy loads in warm seawater I believe changes oil
viscosity and when mixed with sludge acid causes reoccurring restriction or permanent refrigerant flow blockage.