The MethodAPITableListV2 endpoint will return a list of all tables within your account - including any new custom tables you've created, and standard QuickBooks tables like Customer and Inoivce.
To verify - you tried using the tool available HERE and still did not receive a valid response? You only need to input your Method company account name, login email, and password to receive a response.
If you send me an email with your Method account name, I can try running a request from my side to validate everything is working correctly.