We really had fun touring Maui in this little camper. It was surprisingly comfortable and spacious.
We booked campsites at Hosmer Grove and kipahulu in the national park. Officially you are not meant to sleep in a vehicle in Hawaii without a campsite reservation, so we played it safe. (Although from talking to other people and to the renter you can probably get away with finding a quiet area off the road to park at night) campsites at the national parks cost between $5 and $8 a night but book up very quickly, so be sure to get a reservation when they release the dates 30 days prior to your stay.
The car itself was cool. We had a Toyota Siena. It definitely had seen some trips and had some miles on it, but it drove wonderfully and we didnt have any issues with it. One big tip, if you are do rent this car, invest in some elastic window screens that you can get on amazon for around $20. This way you can have the windows open and keep mosquitoes out.
All in all it was an awesome trip!