Hi, we will have the same situation, 3 adults~myself, DH and MIL, and our 2 kids, although we will be by ourselves for the first 3 nights, MIL will be there the last 2 nights.
What you can do is bring a air mattress, I've read that one will fit over by the window, or rent a roll away. We don't want to haul an air mattress all the way from Alaska, so we will probably rent the roll away. Keep in mind that they do charge for the extra adult, $25 per night, and also the rollaway is an additional $25/night. So you could pay the extra adult charge and just bring a mattress if you can do it that way. Hope this helps. Kelly