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The four holes you are speaking about, are they visible from looking down the hole of the floor registers? They would be part of the Artic Package [ Winter Package ] along with heat pads that are attached to the underside of the tanks. Closing those holes won't present any issue unless you camp in the winter and there's any risk of freezing. A pice of foam rubber will work well and can be easily removed if you should need to use them. As for the hole at the furnace, closing that off will push the air through the ductwork in the floor and allow heat throughout the coach. With it open there's no way the air will flow through the ducts. Air will escape through the path of lease resistence such as that opening in the furnace. You may want to install a piece of foam rubber just passed the front and rear register so the air flows out those registers instead of relying on pressure to force it out. The ductwork travels the length of the coach. I know in my case there's 4 feet of ductwork forward of the front register with no openings. Instead of 35 feet of ductwork mine is reduced to about 30-31 ft. There are also registers with dampers available if you don't have that type to help balance out the system.