The problem you describe is not uncommon with this type of mix. Batching order can make a difference. Batching the large aggregate first can help to clean off any of the previous batch that stuck to the blades. Follow that with some of the batch water and then the sand and cement, or you can add the water last. The key would be to have the dry aggregate do its work first.
The hydration stabilizer you are including in the mix should help, but perhaps you need to increase the dosage. If neither of those methods improves the situation, you might try batching bone-dry sand.
In the end, though, you might be trying to force a specialized tool (the transit mixer) to do a job it is just not well suited to do. One of the advantages of RCC is its simplicity--that it can be delivered with regular highway trucks and placed with asphalt paving equipment and relatively simple lay down skills. To take advantage of that you might look into using a mobile central mix plant.