Can a 75 HP Outboard charge a deep cycle battery

G`day Phil,
Only being a self eddicatid bush 12 volt sparky this is what I reckon.
I may well be wrong.
Others may explain it better.

Depending on how long the outboard is running, and at sufficient RPM, its inbuilt charger will replace up to approximately 35 a/h per hour of running.

The fact that the motor is 75hp is basically irrelevant, it is the output of the alternator in the amount of amps to the battery multiplied by the amount of time it`s running.
The amps will vary for several reasons so the "35 amp" figure is the maximum obtainable.

A clamp meter to measure the actual amps will assist, and a volt meter to determine the level of charge of the battery is important.

There are too many variables to give an answer, it will be a matter of trial it and measure.

Scrubby

A couple of hours running during the day plus a genset as backup is verging on overkill. Being a new boat, I wouldn't trust the dealer setup as far as I could throw it, and have an auto sparky check the charge/isolator system off the motor as a matter of course.
I have the same fridge and run a twin battery system in my boat with a manual isolator. I use one battery for the boat, one as a house battery. I only recharge the house battery when underway, and the boat battery is happy. I've never run the fridge like that for more than 2 days, but the smart guages tell me I could go pretty much indefinitely. My 135 h.p. probably puts out a little more current too though.