Depends on the weather, and has evolved over the time I've had my Interstate.
Mattress topper: FoxHillTrading Cooling Touch RV Memory Mattress Topper: Furniture : Walmart.com
("short queen" size is the best fit in my non-extended Interstate, despite the name it's not memory foam)
Sheex: SHEEX Performance Sheets®
(the "nautical" fitted sheet uses a drawstring adjustment and is a better fit for the "short queen" topper.
Mylar space blanket to reflect body heat back up.
Mattress topper (as above)
Sleeping bag: Cabela's Adam & Eve 40° Sleeping Bag : Cabela's
(don't need anything heavier than the 40° model since I'll have the furnace running).