It's My Room! by Robert Munsch, illustrated by Michael Martchenko

From chapters.ca:
Will Matthew ever get some peace in his own room? Not a chance if his relatives have anything to say about it! Matthew's family has a brand- new trailer, which includes a brand- new room for Matthew - a room of his own! Or so his mother says... Sure enough, before he knows it, the relatives start knocking and his room soon becomes a hotel for cousins and aunts and uncles and dogs. Fed up, and sleeping underneath the trailer (which is the only place to get some privacy!) Matthew hatches a plan to get these guests OUT of his room, once and for all! Robert Munsch created this story after having a chat with a young boy in Fort McMurray, Alberta. The result? A funny romp of a story that will satisfy any Robert Munsch fan, especially those that have had to share their room when company comes!