Mine's 81" under the top happi-jak bed with the bottom couch/bed folded down to the sides. No lift-kit on my cart for that reason. My TH IS TT, not a 5th wheel so you may have higher clearance with a 5th.
If you have a dealer close, I'd be taking a trip to measure twice, purchase once.
Even then, I'd be concerned if the placement of the tracks for the happi-jak could vary from install to install if it was closer than an inch to what I needed.
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