Larry N. said:
That figure has to include fuel, water, and sewer, as well as people, clothes, tools, kitchen supplies, computers, hobby supplies, and all the other odds and ends people use -- for a full timer, it's everything they own (other than what's in storage somewhere).
Note that my rig carries 150 gallons of diesel (1050 lbs @ roughly 7 lbs/gal), 100 gallons of fresh water (800 lbs at roughly 8 lbs/gal) and has a 70 gal. grey tank and 45 gal. black tank. My wife and I together weigh near 400 lbs. I probably have over 100 (maybe 200) lbs of tools on board, plus who knows how much in books to read, grill, chairs, small table, emergency kit, spare filters, RV antifreeze, first aid kits, maps, musical instruments, TV/radio/satellite/antennas/etc. Things add up faster than you might think.
Plus, it's good to have some extra capacity (tow a trailer -- hitch weight, kids/grandkids, pets, pet food and supplies). And many options on rigs aren't figured into your base weight, so they come out of that "10,000 lbs of STUFF." It's not just what the wife brings on board.