If you are having trouble with autofocus in lowlight situations just switch over to manual focus. Since it seems like that scene would incompass a lot of area and distance setting the focus to infinity would be the easiest route. If for some reason you can't manual focus properly through the viewfinder (holding camera up) there is a distance scale on your lens anyway. Preset the desired distance and it won't change since it will be set on manual focus. It shouldn't be too hard to judge the distance and f8 would help.