Just start with a flatbed trailer with short overhang. (build, or maybe cut the back off on?) Build a mount for 5th wheel hitch, as low as needed. Would want tongue long enough to keep nose away from tailgate.
If I was building a trailer for this I would want a spread tandem, with no suspension. To turn, would need to slide the tires, so with the weight of 5th wheel on it would tend to go straight. Line up with door, in as far as you can. Unhook, then back the dolly under, lined up straight. Would be best if could go in deep enough to leave dolly under the nose