It is difficult to answer accurately as I do not know exactly what level of accommodation is required. Also the definition of a "settled place" place is unclear to me. However I will give it a go and provide different options.
The highest place with a proper hotel accommodation is the Bokor Hill in Kampot province, altitude a bit over 1 km. The hotel is expensive and luxurious, just recently built. I believe it would be easily possible to do camping at Bokor Hill area as well. However it is not really a proper town or village where local folks would live. When I went there several years ago it was quite an empty place but since then they have upgraded the road and built the hotel etc. so I do not have current first-hand information.
Sen Monorom in Mondulkiri province, altitude about 700 m, is the highest provincial capital. It is a small peaceful town. It feels different there compared to the rest of Cambodia. There are several hotels and other kinds of accommodation available. It is quite cold at night there (cold by Cambodian standards).
It looks like some (western) tourists have gone camping at Aoral/Aural mountain area. I have never been there myself, so I do not really know about that area. I found photos at http://www.tourismcambodia.com/photozone/main.php/aural-mountain/ and some description of a hike at http://distantpeak.blogspot.fi/2011/01/phnom-aural.html.
It seems silly to carry camping gear in Cambodia, as basic accommodation is available everywhere usually starting from 5 USD/night/room and you want to minimize the amount of stuff you carry there. It is hot, troublesome and inconvenient to travel in Cambodia with a lot of stuff to carry. Also even in places which do not have hotels/guesthouses you will easily find families who will accommodate you for a night, if you decide to go really off the beaten path. Just ask around and someone will offer you a place to stay for a night. Basic local food is available at any populated place for 1-2 USD/portion.
There is no such thing as "campground" in Cambodia.
If you want to be able to stay overnight in uninhabited jungle, just buy a hammock with a mosquito net from any town market and carry that. Of course then you would need trees which might not be available at the highest possible places.