If you need some language to persuade a response, try something along these lines... use your own wording, of course.
I am inquiring about your company policy regarding legally armed patrons of your establishment. According to the sign on your facility legal carry of handguns is allowed. However I was asked to leave your place of business when I entered with my family legally armed with a handgun.
If your company policy is to disallow legal carry of firearms then my only complaint would be that your signage on the property conveys quite the opposite and should be remedied to prevent others from being embarrassed by its misinformation. It is your legal right to dictate firearms policy for your private establishment.
(Here's the kicker)
If, however, I was singled out for another reason then I take great exception to being discriminated against in a public forum. In this case your representative took it upon himself to extend his, or his supervisors personal feelings toward me and my legal, Constitutional rights. This to me is unacceptable and something I would pursue with an organized boycott of your establishment, written letters to your corporation, the state and local media as well as the possibility of legal action.
Reserving the right to refuse service to anyone does not include racial or religious reasons. You can refuse to serve me a meal because I'm black but I'm going to sue you and destroy your business in the media.
If the company policy allows for legal carry then you should be allowed to legally carry. If they don't want you to carry openly then the sign and policy should both specify.