I've done this heaps in my Jeep Cherokee. I took a two month rod trip across the USA (some bits of Canada too) and I alternated sleeping in my Jeep with getting a cheap hotel room every few nights. KOAs, forestry campsites, private campgrounds, Wal-marts, Flying Js, dry camping on BLM/Crown land, all sorts of places.
My g/f and I also did a three week road trip through BC and the Pacific Northwest on our way to/from a friends' wedding where we did the same thing.
With the seat back folded flat and the seat bench removed entirely (pops out with one little button in a Cherokee), I'd store our bags directly behind the front seats, lay sleeping pads over the cargo area and bags, stash the cooler in the front passenger seat (the rest of the gear was in the ski box). With enough sleeping bags and blankets it actually was very comfy - but to be fair, we're both wee short-arses.
Occasionally in a busy Wal-mart or truck stop I'd lock up tight. In most cases it went like this: on warm nights we'd leave the rear hatch open and drape mosquito netting over it. On rainy nights we'd do the same with a big tarp.
I'd love to have a kitted out expedition vehicle, but the ol' Jeep does just fine for my dirt-poor budget.
