The "fixed-voltage" (13.5 volt) mode is meant for using the device as a power supply, essentially bypassing all battery charger and conditioning functions.  Although 13.5 volts is indeed a float voltage, most battery chargers limit float current by "pulsing" or other current limiting means.  Since the fixed-voltage mode is meant to be a power supply, current limiting is not going to happen to the same degree as a charger in float state.  Therefore, there's some potential to harm your batteries when left in this mode for an extended amount of time due to overcharging (trickle).  Really no concern as you won't be running your generator long enough, but more of an issue here if you've chosen this mode when plugged into shore power for long unattended times.

(If you leave the device permanently in fixed-voltage mode, your batteries will not go through the proper charging stages.  Their life will be shortened and will never be fully charged).

I'm a bit surprised that the fixed-voltage mode does not trip your breaker just a readily as in battery charger mode.  This may suggest that there's some current limiting going on.  Regardless, as a compromise select fixed-voltage mode until the batteries come up to a point where the generator can support 3-stage or 2-stage modes.  When you do switch to 3-stage or 2-stage modes, the batteries will then go through the proper charging stages.  Put a note on the generator kill switch as a reminder to return the switch to the proper position.