In general: The van was great, it isn't the newest one (like you see on the pictures) but it did everything it was supposed to do, everything worked perfectly fine. The interior was very nicely done with a lot of attention to details! Everything you would need for a trip, you can find it in the car - from a shower to a sink, a camping gas stove and basic cooking utensils. The host thought everything well through!
Cleanliness: everything was great as well - the van was perfectly clean, you couldn't find a hair or anything else from a prior guest! No bugs at all, but since you will have food in the car, I suggest you don't leave any open food bags in the car and close the doors as soon as you don't need them to be open anymore. We didn't have any bugs in the car, but we did everything we could to avoid them (no lights in the car when it's dark, immediately closed the doors, no food in trash, no open food bags etc.).
Sitting/ sleeping: the bench seats were nice if you want to relax for a bit (but as far as I remember, there weren't any seat belts, so it's not a good idea to sit there while driving). Mounting the bed was nicely explained by the host and very easy to do. The bed was very comfortable and we slept better than expected. You don't have too much space, but it's enough to get a good night's sleep when you're 2. It can get very warm during the night, but if you open the rear window on the right side, you should be fine - it got even so cold during the night (end of July) that I had to take the blanket (there were new bedsheets, 2 pillows, a ''normal'' blanket and a warmer one).
Where we stayed: we stayed at a campground. Looking for information on where to camp can be confusing on some Hawaiian websites, but the host was very helpful and gave us a lot of information on that matter. Since campgrounds are limited, I suggest you book your spot well in advance!
Host: the host was very nice, explained how everything works very well, answered within minutes, check-in and check-out were very easy. Nothing to complain! He did a really nice job! :)