RCS - Disgusting Customer Service and Arbitrary Statement Changes

We have a number of accounts that are either controlled / underwritten by RCS. Over the past month, we have come across discrepancies on all statements across store cards. We have spent a lengthy amount of time on the phone to try and understand the shifting of balances, the arbitrary interest rates charged, and current account standing. They have remained extremely unhelpful, blocking us from attempting to escalate the issue and have not clarified the arbitrary information to date. Their customer service is non-existent - it seems like RCS cares more about overcharging so that their books look good, and not about the customers who are trying to understand arbitrary charges, unfounded balance and budget shifts, and extra charges. Please can someone advise on how to resolve this without legal intervention?