$28 million is the house's value, not the unpaid bills. Also, I expect Zsa Zsa wants care better than Medicare pays for - such as a private room, personal nurse, doctors that work on her schedule rather than getting around to her treatment when they have time behind all the other Medicare patients.
The article said that the medical bills were $21,000 per month plus $35,000 a month for upkeep on the house itself.
$28m invested at 2.4% after tax would cover it.