Standard fifth wheel hitch systems are designed to attach to the truck's main frame rails. The Ridgeline's unibody frame and in-bed cargo compartment are not compatible with conventional hitches.
Floyd reported one person fabricated their own custom hitch that cleared the compartment, attached solidly to frame members, and was towing a Scamp
19 with a G1 Ridgeline, but I couldn't begin to tell you how to do that. Such a set-up has not undergone any kind of rigorous engineering analysis or field testing as vehicle and hitch manufacturers routinely do for liability sake.