"Ippaku suru" is a verb, meaning "to stay overnight". However, in the phrase "ippaku suru heya", "ippaku suru" is actually an attributive clause whose subject(I or we) is omitted, thus, "ippaku suru heya" means "a room I/we (will) stay overight".
As for "isshūkan heya", "isshūkan" doesn't modify "heya". Grammatically, these two words have nothing to do with each other. I'm sure there MUST be a verb after "heya" in the sentence. "Isshūkan" performs as an adverb to modifiy the verb.