I just did a test with an amp meter. My built in converter charges a low, almost dead, battery at about 8 amps, my Yamaha 2400is at about 8 amps and a separate battery charger made by champion about 8 amps. These measurements were taken right when charging started, the current tapers off as the battery charges. So they are all about the same. I just stick with the built in converter.
2007 FJ cruiser
Tekonsha Prodigy Brake controller
Yamaha EF2400is generator