just wondering if RV's are allowed to park at the Waimanalo Beach Park/campground. Wouldn't need a hook up.
Not sure if there are any RV's on Oahu to rent. There may be some VW camper vans available to rent.
Oahu is a small island. There are a couple of driving loops that you can do from Waikiki (or mostly anywhere on Oahu except the west side) in 3-5 hours each depending on stops and duration at the stops.