。
She had the spacious room all to herself.
Literal
She [topic-は] that spacious room [object-を] hogged.
独り占めする describes the act of monopolizing — claiming exclusive use. Built from 独り ('one person, alone') + 占め ('occupying'), the verb captures the slightly playful or critical flavor of someone keeping a desirable space, resource, or attention to themselves. The plain past 独り占めした reports it as a discrete completed event; an alternative phrasing 独り占めにしている (using にする + ている) would frame it as a maintained ongoing state. Common in casual speech for any 'X-hog' situation — bathroom, TV remote, dessert.