One good and inexpensive way to increase the effeciency of your APU is with insulated window shades. My wife made ours out of the silver bubble type insulation that is sold at Lowes or Home Depot. It comes in roles and is used for wrapping hot water tanks etc. She cut it to fit the windows and sewed cloth around the edges to keep it from fraying. She used masking tape on the edges of the side window pieces and that has held up just as good as the cloth so far. It keeps the truck much cooler then the curtians that most come with. It is much darker inside so sleeping in the daytime is easier. It also blocks the lights from the parking lots or moving trucks at night. Layoutshooter