Not very much.
Our house is over 3,000 square feet with a basement and an attic. There is another attic that is accessed through a linen closet but that would mean combing through the items on the shelves and removing the shelves. That is too much trouble. I have never actually seen inside this secondary attic. It intrigues the children and there is always the hope that there is some cool antique artifact just waiting to be discovered. In my house there is lots of space to hoard precious items.
Our RV is not quite 300 square feet.
Each child will have a cubby that is approximately 2 cubic feet. The kitchen has 2 cabinets, 3 small drawers and 3 wide drawers. I would count the microwave as a storage space but after the strawberry incident that is not a wise. Computers and educational items take up more space. Under the bed we can store coats and heavy blankets. Bedding will have to go somewhere, and towels, a first aid kit, bug spray, candles, chairs to sit outside, extra shoes, umbrellas…
What am I going to bring? I have no idea where everything is going to go.