Unfortunately it is difficult to get a "best solution". I've fitted 40+ cameras to Ducatos and mount the camera above the rear doors facing down. The purpose of this is so drivers can back up to loading ramps, hopefully stopping before they hit. These vehicles have rear steps which are stowed under the bumper, and we have had a lot damaged in the past from concrete loading bays leaping forward and hitting the vans!! Also it gives a clear view if pedestrians are walking behind, usually covering an area wider than the van and 3m plus behind.
The disadvantages of this camera location are rear perspective,ie. not a great view behind if you want to keep an eye on following traffic, or a trailer being towed, and most importantly, no height cover, ie. you may not see overhanging verandahs, tree branches, etc.
The second option is to face the camera back (not down). This will give you a larger view behind the van, and is better for traffic view (if switched on when going forward, which most cameras can do). If mounted low down, any obstruction on the ground can be seen.
The disadvantages of this are sunstrike-if the camera is horizontal you are more likely to get morning or evening sun shining directly onto the camera, which destroys any chance of seeing anything; harder to keep lens clean; if mounted high up, you may not see children or wheelchairs behind you.
The best option is to mount and wire the monitor, run the camera cables, and use double sided tape to try different camera positions.