Fantastic way to see a ton of Kauai within a short time frame!
We camped at 4 campgrounds, sleeping in the 4runner each night. The mattress was 2 layers of memory foam that was perfectly comfortable for 2 people, and I had just enough room to fully extend at 6'1". It's not entirely flat, so park strategically. I'd recommend a sleeping bag liner for hygiene/warmth as only a thin blanket is provided. The campgrounds cite no sleeping in cars, but we were not alone in doing so and no one ever confronted us.
Walking 5 min to the off-airport lot was fine, Shelly provided clear instructions. The 4Runner gave us minimal issues throughout our trip. We drove ~200 mi throughout our stay and averaged 17-18mpg, so factor the $5+/gal into your final budget.
All of the equipment comes in a large plastic bin. The two of us packed lightly and had enough room, but driving with 4 people or extra baggage is infeasible with the folded mattress taking up the entire trunk with chairs + tent.