Correct on Unlocked.
Standard Transmission? Then just Neutral.
We have a Brake Buddy. AKA Brake-in-a-Box. Device sits on the toad's floor and when activated it pushes the brake pedal with the driver's seat as backup. It develops enough force with its internal air compressor that you MUST bleed the toad's vacuum booster down before pulling off. The process is to get all hooked up, engine off, then pump the brake a half dozen times to get rid of the vacuum stored in the booster. THEN connect the Brake Buddy. Otherwise that first stop is a doozie. The BB will apply all its force on top of the boost in the the toad's system. A moderate stop in the RV will lock the brakes in the toad.
Be sure the front suspension in your Ranger is TIGHT. If it's worn out, needing tie rods, etc, it'll be back there trying to wander, and it'll try to make the back of the coach wander. Likewise there can't be looseness in the towbar, baseplate, or the RV's hitch. Those Drop/Raise Drawbars can wobble side to side enough to disturb the tracking of your toad.