You make a floor for them and add whatever furnishing you want. Our floor for the large one comes apart in 6 pieces. Some folks just you furniture that has pedestal style legs.
Hubby decided he wanted to go fishing with his brother and dad down in TN, and took the only reliable vehicle we have. Afraid to drive my car that far(2 hrs each way) because it bogs down when it is running for a while... Think I need another catalytic converter on it.