I bought a 950w 2 stroke with a 12v charger built into it just for this purpose. its small,light & stores in the engine room. Think it was £70.00 from Makro's. a bit noisy but charges batterys quickly.
I see you keep your generator in the engine room. I do too. Often wonderered if its safe to keep one in there, petrol and all that. Anyone know what that may do to the insurance cover if any thing happened?
Be careful with generator choice because the power to run the charger may be more than you think. There are power/efficiency losses between 220v AC and 12v DC and unless the Mastervolt is a very recent one these losses will mean a bigger genset than you might theoretically think. Some of the latest chargers have a means of dropping their output to 75%/50%/25% of maximum for this reason and for low amp marina outlets.
How about a fuel cell ?? Totally silent and provide a permanent topup for batteries. Somewhat expensive initial outlay though. The charter companies are installing them instead of running engines for charging.