You list DRY weights for your trailer, so really you're probably looking at closer to a 7000lb+ trailer with 1000lb+ tongue weight.
Torklift's Superhitch system is designed for use with a WD hitch extended as much as 48", and still has a 1200/12000lb trailer capacity at that length IIRC.
Reese makes an extension tube that does up to 34", but near as I can tell it is only rated for 600/6000lb trailers with WD.
Beyond that, the extensions you can find in stores, or from online sources like etrailer.com, are only rated for 350/3500 or 500/5000 at most, not for use with WD hitches.
Really, the only commercially available manufactured solution with the capacity you need is the Torklift Superhitch.
Truck camper guys that don't go with the Superhitch usually build something themselves or have it fabricated professionally. This requires some engineering know-how, though.
