Actually, I have never heard of any legal agency or company which offers such services to guide gamblers when casinos ask for very specific documentation like those papers you are being asked for. I believe, you don't have anything to hide about your income, you should go for it if the casino is a reliable one and one with a very good track record within the industry. If you want to have some professional assistance on whether you should or not to submit such information to the casino perhaps it would be more appropriate if you hired some lawyer with experience dealing with gambling services.
That is the only idea which comes to my mind.

I hope you have good luck and you do not find further inconveniences.