Goose neck, 5th wheel connection, Long bed, short bed truck

Good luck finding a GN RV. They may exist but are extremely rare. Converting one isn't advised due to differences in frame.
For a trailer that heavy I think you would want a dually.
I would always prefer a long bed truck. Truck's are generally for hauling and a bigger bed fits that need. Also, as you are aware, it's simpler to pull a FW trailer with a long bed. Often, long bed truck's have larger fuel tanks than their short bed equivalent.