I dont have a toy hauler, but the few times i have had my HD in the bed of the truck i would put a 4x4 and 1x4 block of wood under the frame at the engine. Then hook my straps to the fram at the crash bars, the wood acts as a stop so you dont over compress your forks.
Also you didnt mention which model HD, but i know my ultra has rubber mounted handlebars so i would strap to them.
My fathers TH has tracks in the floor which allow for hook points, he also puts some wood under the frame and then straps till the bike is sitting on the wood.