I would look for a hitch-to-hitch towbar (I've seen them in use for moving used cars around in pairs). This would have vertical flexibility at both ends, and you might find one long enough to deal with the range of vertical movement at the rear of a diesel pusher.
My problem with your single hitch ball idea is that there will not be sufficient vertical flexibility to deal with how much the rear of your motorhome moves on bumps and dips found at driveways, in campgrounds, and even at intersections of heavily crowned city streets.
From the rear bumper of your motorhome to the front axle of your tow, the difference in height can easily vary by more than a foot. Commercial towbars accommodate this by having 30-50 degrees of free vertical movement at each end of a fairly long bar. Even with this much to work with, towbars or baseplate mounts have been known to fail with a single excursion outside the design range, when something hits the stops and one tries to lift the towed vehicle into the air or slam it into the ground.
With a single ball at one end, the towing connection solid at the other end, you will be regularly exceeding the range of motion of a ball hitch, and possibly the strength of your hitch receivers, if you hit that situation where you are trying to lift half the weight of your truck off the ground at 50 mph on a construction dip. This happens even when using well designed hitches, and tow bars, but without flexibility at both ends of the towing equipment it is going to happen real soon.
