My ramp is covered with grip tape. I've loaded the bikes in the rain more than once and can say that it is anything but slick.
I use a couple of these (similar) and put them under where the rubber bumpers are on the door. I usually have a 2x4 in the back of the truck that I will use if I need a couple of extra inches.
Last time out however, the distance to the ground with the door level was about 24" so I didn't have enough reach. It was a great spot to sit also, over looking the river.
http://www.hilltoprv.com/parts_store...5424926048a12a
This made me start thinking about using steel cable and turnbuckles to build a support from the frame to the door. I have a couple of friends that have this system on their TH.