Overall it was great to have the freedom to drive around the island in a van. We had the van for 5 nights and it drove well and was pretty comfortable aside from the bed being a bit short for my 6' partner. It was great to meet Amber and have her show us everything. She was also very responsive during the trip when we lost power for the inverter and fan. It did end up being a bit more stressful than I anticipated, and I would do things a bit differently next time, so I'm including my suggestions here but I realize that some of this just comes with the territory.
Though you may see a lot of vans parking at beach parks, if you see the yellow gates at the entrance, know that you will get locked in until they come back in the morning. Don't assume that you won't get locked in just because the gates are not locked by the posted time. They came around 8:30pm to lock the gates, which also means they may not be back at the posted time to let you out. We narrowly missed getting locked in and would have missed our SCUBA dive the next morning if that had happened.
Despite other reviews, you cannot stay at the visitor's center at Haleakala, or anywhere in the park at Haleakala except the Hosmer grove campground, even if you have a reservation and even though the park says it is open 24/7. We left early to make it up before dark and after sunset, which was beautiful, the ranger came around to all the vans and told us we had to leave or get ticketed. We ended up parking outside the park along the side of the road, which was 4,000 feet down the mountain, and about 30-45 minutes of driving depending on traffic.
Staying at Wai'napana State park was the best experience because we had somewhere safe to be and it is a beautiful park. I highly suggest staying there and getting up early to do the rest of the road to Hana that is past the campground before other tourists reach it in the morning. That said, even taking two days was not enough time to do everything on the road to Hana.
We also enjoyed staying at McGregor point, which was pretty abandoned overnight but there were some people coming in and out who did not appear to be tourists. We stayed at the lower parking lot and watched the sun set over the bay and Haleakala from the bed in the van with the doors open and it was great.
Other than those two spots, we ended up parked along a road the three other nights, which wasn't ideal for me, but it worked. We even parked overnight on a side street in Lahaina because none of the parking lots allowed for overnight parking.
If you plan on doing a lot of cooking, you may need to buy additional pots and pans and you will need more gas for the camp stove. We only used the stove 4 times and on the 4th time boiling water in the tiny pot we ran out of gas before the water boiled.
The cooler does not insulate particularly well and ice melts quickly. It does not have a drain valve so watch that your food isn't getting submerged throughout the day. We ended up throwing out some of the food we bought. I suggest using 2 small bags of ice and you will need to buy more every day.
If I were to do it again I would try to have a more solid plan for where I knew we could park overnight. I would plan on the extra time needed to get groceries, ice, car & stove gas, and find parking every day. We didn't buy groceries every day, but I don't think we could have gotten away with only one grocery stop due to the amount of ice needed to keep the cooler cool. We sometimes used the small beach cooler to store a few things for a while with some ice while enough ice melted to put them back in the main cooler.