I loaded a non-running 750 into truck bed using one friend, harbor freight aluminum ramps, and backed up to a slight hill (maybe halved the distance to the ground). Couldn't have possibly done it alone. Could have used a winch to pull it up the ramp, if things had gotten desperate, but like KnifeMaker said, the angle of the ramp and the tailgate will cause the bike to bottom-out hang up, and most likely fall over.
I did manage to load a 750 into a black widow hitch carrier from level ground by myself, but it was a functioning bike and I was able to feather the clutch while walking beside the ramp. Was uncomfortably dicey, so I had a friend help push it up the ramp without motor assistance the next time I loaded it.