Could the Ronald McDonald House, a 503(c) charity give a federal tax deduction for rooms donated when the House is full. They do fill up. Medical opinion see healing benefit in having family and friends near the hospital where the children are patients. This could also help the motels, hotels and their staff keep their jobs. If possible, what would be the method for calculating the deduction? Rooms that go unrented earn nothing.

If you are asking whether a lodging place can TAKE a tax deduction for rooms donated to a charity, of course they can. The value of the deduction would be the standard room rental.

... Rooms that go unrented earn nothing.

Bananas that are over-ripe cannot be sold. Yet a store could donate just-ripe bananas to charity.

Similarly, a night's stay at a hotel, until that night has come and gone, have a market value. The hotel which donates that night's stay can deduct that value.