I have encountered some "full time" RVers sleeping in their cars. Finances are usually to blame.
My wife and I and two cats full timed in a medium sized truck camper without slides. We had the basic comforts and then some: full queen sized bed, refrigerator, 3 burner stove, toilet with wet bath, dinette, hot water/furnace, AC, and even a microwave. We had plenty of storage including an overhead 2x2x6 foot cabinet and the back of our crew cab. Going small is all about mobility and traveling. We drove 30,000 miles the first year and visited countless National Parks, State and Provincial Parks and other attractions. Others think of full time living more along the lines of going to a warm location and spending the winter in an RV park.