We rented the smaller houseboats many years ago and it was a blast. I think Cumberland lake levels are still lowered for dam repairs but it's still quite useable. You would want to check around the ramps you intend to use to make sure.
One thing to watch in towing jet ski's to make sure you pinch off the water intakes or whatever is recommended for the model. A friend of mine neglected that with a standup Kawasaki and when he arrived at the dock it was an anchor at the end of a 30' rope, below the houseboat! Worse yet, he had borrowed it from a friend for the week. Oops.
In terms of fuel, we tended to move once a day to the next spot and explore with the ski boat and Seadoo. It was very easy to find calm branch sections to ski and play in, off from the main body, so we didn't actually need to go too far. Definitely a good time.