I have used my battery charger powered from the generator (built-in) to charge the batteries. It does a much better job faster than the converter. If I manage to let the batts get way down, I use the 10a setting for about an hour, then drop it lower.
'course, when using the gen, it is also charging the batts...
Not being the sharpest electrician around, I would use the high setting for a period of time, the go to the next lower (6?) before using the 2a setting.
You should be able to use the 2a for as long as you want.
Be sure the battery fluid is at the correct level before charging...
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired