Remove the outdoor shower head. The pipe for that is a 1/2 inch thread same as shower tube. I went to Home Hardware and asked for a 1/2 inch to garden hose adapter. (see pic below). The adapter also fits on the end of a RV water pumps so I have several.
Connect the shower pipe to the shower hose and then the adapter. You will then be able to connect your garden hose.
I also use the shower pipe and adapter combination to connect the shower hose to the turbo flush. That way I can rinse my black water tank at the Ontario provincial park dump stations. Since you are an Ontarian you know what I am talking about.
My conversion is in the trailer at the storage so I can't send you pictures of the little pipe I carry around. PM me if you need more info.