I don't recommend under-bumper mounting as the lights will be subjected to just that much more road debris. You'd be amazed how much damage is caused by road grit/stones kicked up by the traffic ahead of you.

Also, try to make sure the lights are mounted to something solid. If your bumper vibrates at all when you go over bumps, the light beam will "dance", and will drive you crazy.

One option I've used in the past with auxillary lights on a PV is to install them on brackets that attach to the radiator surround sheet metal instead of the bumper. This is significantly more stable, and also places the light as far from the front of the car as possible to avoid damage from other driver's park-by-braile technique. You can even mount them behind the grill and cut holes in the grill mesh.