The above answers pretty well cover everything, but I'll chime in anyway.
Our furnace vents don't get covered with the slides in and, when traveling during Winter coming from Goshen back to warmer climes in 2010, we ran with the furnace thermostat set to 50. Sure made a difference. After we started doing that, I realized I had been previously towing a block of ice. Just about literally.
Made a heck of difference.
Betty