What can I do if I had a major fire at my house and my insurer is not paying me what I am fully owed?

If you believe that your insurer is not fully honoring the policy--e.g. paying less than they are supposed to under the policy--you can sue them to force them to pay more. There are attorneys who specialize in suring insurers who don't live up to their obligations; you should consult with one, bringing with you the policy, correspondence from the insurer, and back-up as to costs and values, to see whether you have a claim, what it might be worth, and what it might cost to pursue it. Note however that if the replacement cost part of the policy applies only to the home, not the contents, then steep deductions for older items may be warranted. Also, replacement cost for many items may be lower now than you paid before (e.g. for many electronics, which have come down sharply in price).