When I first had my RX400H I found rear visibility from the RX400h is also not helped by the headrests on the centre rear seat which is right in your line of vision if you are reversing in the traditional matter - i.e. looking through the rear window
Looking at the rear view mirror is also pretty much a waste of time. I had rear (non Lexus) sensors fitted to bleep if I was getting too close to something - and they have been a godsend.
However I'm now fully adjusted to reversing on camera alone ( you can increase the contrast on the screen) . I also find there's plenty of light at the rear if you have your brakes (and rear brakelights) on when reversing.