Before you lay down the money for a converter, I'd have the batteries load tested - it sounds like you may have a cell going bad. It goes without saying that electrolyte levels should be checked.
If you do have to replace the converter, I'd recommend Progressive Dynamics. The specific model would depend on what you have presently for a converter.
Rusty
2014.5 DRV Mobile Suites 38RSSA #6972
2016 Ram 3500 Dually Longhorn Crew Cab Long Bed, 4x4, 385/900 Cummins, Aisin AS69RC, 4.10, 39K+ GCWR, 30K+ trailer tow rating, 14K GVWR
B&W RVK3600