Martha Wayles Skelton Jefferson died at Monticello on September 6, 1782. No known eyewitness accounts were written at the time of the event. Therefore, we cannot precisely determine Mrs. Jefferson's sickroom.
The best source we have for a description of Mrs. Jefferson's death comes from Martha Jefferson Randolph. Thomas Jefferson's eldest daughter, nearly ten years old when her mother passed away, composed the following account many years afterwards:
For four months that she lingered, he was never out of calling; when not at her bedside, he was writing in a small room which opened immediately at the head of her bed. A moment before the closing scene, he was led from the room almost in a state of insensibility by his sister Mrs. Carr, who, with great difficulty, got him into his library ....1
From her daughter's account, we know that Martha Jefferson was bedridden for four months prior to September 1782. It seems unlikely that her sickroom would have been upstairs — the upper rooms would have been uncomfortably hot during the summer months. Probably, Mrs. Jefferson occupied the downstairs room that would become her husband's bedroom. (Note that the alcove bed did not exist at that time.) The "small room" where Jefferson kept vigil was most likely the room now called his cabinet.
It is important to remember that, in 1782, the layout of Monticello was different from its present-day configuration. Originally, the library was housed upstairs. Martha Jefferson Randolph's comment that Jefferson was led from "the room" to "his library" is therefore confusing. It seems unlikely that Jefferson's sister could have "led" a full-grown man on the verge of collapse up an extremely narrow stairwell. Martha Jefferson Randolph may simply be describing the space in terms of "Monticello II," rather than the Monticello layout of 1782.
Jefferson's library would in fact eventually be the next room over from the room he was probably staying in during his wife's illness, but at that time it did not exist. Mrs. Carr may have simply taken her brother to the same room that his daughter said he was writing in during his wife's illness.