I have a B&W Tow n Stow hitch. I do not unhook the trailer because I do not want to loose control of it. I take the pressure/weight off the hitch with the tongue jack. Pull the two pins on the hitch for hitch height. The hitch will now slide up and down with a pin built into the hitch that stops it from coming apart, you would have to see one to really know what I'm taking about. Jack the tongue up all the way or so that the ramp is level with the floor of the trailer. Unstrap and remove bike.
I you already have a hitch another way is to run the trucks rear wheel onto 3 pieces of stacked 2 X 12 made into a ramp. Either way you will not loose control of the trailer as it remains hitched to the truck.

 

The old school method on the road with limited tools was to go to a bank and get a roll of nickels and stick'm between the links, or so I heard.