in addition to erythritol a number of the low-carb chocolate makers will use one of the long-chain sugar-starches like polydextrose or inulin, which aren't sweet, but are possessed of many of the properties of glucose and fructose, respectively, and then a supersweetener like sucralose, monkfruit, or splenda for the actual sweetening power.
you can also combine these with xylose or erythritol / other sugar alcohols (but definitely avoid maltitol).