You are correct that you have a legal right to your assets. If the accounts are in your name and not your husband's, I don't see why USAA would refuse to transfer your assets. Even if they were in your husband's name, providing a copy of his death certificate and will showing you as heir, should be sufficient evidence for USAA to provide you with access to your assets. Unless a lien has been placed against your accounts for unpaid debts, I do not understand why they would refuse to give up your assets except that they obviously do not want to lose your business.
Unfortunately, there is really no advice I can give you expect to contact USAA again. If they are persistent in refusing to give you access to your accounts, I think you need to get your attorney involved.