What's the model of that converter?
It may be a single stage... or a multi-stage model that is difficult to get it to shift into high gear.
I believe a Progressive Dynamics PD9260 (60A) is rated at 1000W going full out.
As others have mentioned, that genny of yours wouldn't keep up with a 60A PD nor a multi-stage 55 Amper. If the bank is low, even a 55A single stage should run at full rated power. So something's amiss, here.
Regardless, I echo the sentiment that your battery indicator is likely worthless. If you want to use voltage, the bank needs to be free of any draw and rested for at least a half hour and ideally an hour or better.
A glass hydrometer and thermometer are the best ways to determine true state of charge. A glass hydrometer can be had at O'Reilly's for about 5 bucks. Temperature can be estimated to get you close. Then a state-of-charge chart like the one below will give you the real story.
