Health insurance denying claim for deceased patient. Any advice?

Linda, was your FIL on Medicare? It seems unusual that hospitalization would not be paid. Can you clarify if it is Medicare? Alternatively, your husband might send a letter (return confirmation requested) to the hospital and copy to the insurance company. Letter can clearly state that you do not understand why the bill was denied and without further information, there is nothing you can do. Then just file the bills until something more comprehensive comes your way.
Hospital and doctors can get the bills out faster than insurance companies can process them. Meanwhile, bills may be forwarded 'for information purposes' to the patient/estate. If your FIL passed recently, I would just wait it out.
AND sorry about your loss, this frustration on top of it is discouraging.

Who is the Trustee of the trust? Do you have documentation of that?

I think I'd send a letter to the legal department of the insurance company and ask how to establish the fact that your are authorized to speak with them.

Apparently my FIL as an individual and the trust are two different entities. We can send a Small Estate Affadavit, as my FIL, the individual (and the recipient of the bills), leaves an estate less than the required cut off. Yes, Medicare should be covering these bills as the medical care was quite necessary in all instances. We've been paying the co-pays right along. I think we'll find some lack of information provided to Medicare by the insurance company. I had to get really stern just to get a fax number from a supervisor to send the information to - they kept telling me to use snail mail. I'm just really cranky about it all, as circumstances (and family) have kept me in "to do" mode and I haven't had time to even grieve.

I really can see where ur coming from. I wanted to get Moms bills cleared up before Medicaid took over. Get the affidavit. You won't have to show where the money goes. When it's all said and done, the hospital may have coded it wrong. The Medicare paperwork should tell you why they didn't pay. Then you need to look at what the supplimental says. I always wait till I get the bill to find out the final balance. A lot of adjustments are done. You say insurance giving the wrong info. Normally Medicare is primary and supplimental is secondary. The hospitals, labs and doctors bill Medicare. Medicare pays what they feel is reasonable. When the hospital, etc gets the info, they then bill the supplimental. Who pays the balance or partial. Coding incorrectly is usually the problem. Meeting the yearly deductible is another reason Medicare and supplimentals don't pay. The Medicare statement will show in letters A B C the reason why they didn't pay. Sometimes the hospital billing department just have to refile. That happened with one of Moms bills.