It could be a number of things. The best course might be to take it in, but there are a few things you can do with a multimeter to find out why the converter is not charging the batteries. But, lets try to narrow it down a little before you take up too much time.
I should have asked this the first time, but except for charging the batteries, is anything else in the trailer (lights, fridge, heater, outlets, micro, A/C, slide) working when plugged into 120?
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD