I have found that a four-person tent with a good fly and enough head room to stand up in makes a luxurious car-camping apartment. It sets up quickly and is about x100 cheaper than a travel trailer. Plus I can get away without having a big car for towing, saving $$ on gas that way.

One key to this setup is getting a good comfortable camp bed. Mine is a 4" thick air mattress with a closed cell blue foam pad on top for insulation, and a down pillow for my head.