I seem to recall from the days I had a property and casualty license (many years ago) that your fire claim is between you and the insurance company. If they choose to go after another party that is up to them, but they are supposed to make you whole, to the extent of your policy limits, without waiting for the other party, in this case Norcold.
It may be that they feel they have a better chance of recovering if they can get the Norcold inspection done before repairs are started. Ask them if they will waive the deductible as compensation for your waiting, then suggest you will have a conversation with the state insurance office if they insist on delaying. that may be far more effective then waiving a lawyer letter at them. They have LOTS of lawyers, there is only one Insurance Commissioner per state