The 3rd generation grand caravan would be my choice for a single person, about a 96-99. They are so much less initial purchase ($2000 will buy a nice one) that even if they only get 22 mpg you will almost never make up the purchase price of even a used Prius that you can't camp in. These will also travel dirt mountain roads, beaches, snow, ect pretty well. The 3.3 or 3.8 v6 were the most reliable and common. The minivan will stealth camp better then anything. The only thing I personally would customize would be to add a real rv heater for cold nights as I just don't trust "tent" heaters. That and a 2nd separate battery for the heater fan and whatever other comforts you might add, then you never drain down your van battery. By the time you add a battery box, heater, and a small underbody propane tank you will be over $1000 more but this would my heater choice
Van Cafe: Vanagon Parts - Propex Propane Heater HS2211 Kit | Heater
It mounts underneath the van. The pre 4th generation caravans didn't have any fold into the floor seats so they have more room underneath or on the later vans the rear seat storage well would also work but I would cover it and completely seal it off from the interior.
The good thing with stealth camping is you don't always have to pay for a real camp spot. Walmarts, rest areas, some pullouts along the highway, just about any forest service road, etc. Free.