2011 Holiday Rambler Endeavor - also had no "factory" spot to store the slinky.  Took 2 pieces of 4" plastic pipe and set them in the corners above the propane tank.  Propane tank has door just in front of wet bay, bottom is open to atmosphere so things will dry, and the 2 pieces of pipe fit perfectly in the curve above the center line of the tank towards the ceiling.  The pipes lay in there perfectly - make sure you block off the back of the pipe so the hose does not fall out the back,  and a large plastic wall anchor through a drilled hole in the front edge of the pipe catches the lock ring of the slinky perfectly and keeps it from coming out the front.
If you can't visualize what I am talking about and your propane tank goes cross coach, just take a look in the compartment.  The PVC pipe should fit in the upper right and left corner of the compartment laying on the tank and against each side wall.