Lowes' and Home Depot... I remember the day when, during my initial "get-it-liveable" construction phase, I didn't have to go to the hardware store for the first time in a long time...
I smoke, and I couldn't smell asphalt if it was smeared on my cigarettes!
I used thick poly sheeting for a vapor barrier, running strips of it along the walls down to over lap a poly "floor pan" that was a single sheet as long as my floor and that ran up the sides about 8". It has 1" styrofoam and 3/4" ply on top of it. No complaints here.
I have an imported Chinese tile floor on top of the plywood (the self-stick vinyl tiles said "made in China" on the back -- that's imported Chinese tile, right?) I bought the cheapest tile I could find because I was running out of $$. It was $0.38/foot. All in all, it has worn well. It DOES NOT do well in direct sun. It bubbles up as the tiles expand and push against each other at the edges. Over time that is less and less of a problem, but I do try to keep sunlight off of it. It also discolored very badly underneath a black rubber door mat that sat in the sun at my back door. But it was cheap.
You have to remember, every thing you add to the floor cuts your interior height. Right now, if I had to replace my flooring, I'd go with very good sheet vinyl. I might go with laminate, but it's would end up costing 1/2" of interior height.
Keep cleaning in mind. A nice smooth floor is easy to sweep clean in a bus, and if you go in and out at all, you're going to be tracking something in.
The nicest thing about living in 200 square feet of space -- you can mop the whole floor with Windex and a few paper towels!