My dog sleeps about 20 hours a day. Wide awake when there is food on the go though. She is deaf, going blind and has cancer but she is the sweetest animal you could imagine. I have never heard her bark, she is disinterested in other dogs other than to give them a quick sniff if they come up to her. Otherwise she is content to lie with her head on your lap. She would be a perfect candidate for any rental unit.
I don't know about Great Danes but Greyhounds need about 20 minutes of vigorous exercise per day then they are content to lounge about for the rest of the day.