We did it on our 2013 Sabre 36QBOK, so it might be a different slide mechanism.
We first did it with just a 1x6 to span over the slide rail framing with “under bed storage boxes”. I later fully enclosed the area with extra wood and plywood.
Worked a treat for us while fulltiming and moving every 7 or 8 days for 2 1/2 years.
2018 Ram 3500 DRW/crew cab/long bed/4x4/4.10 gears pulling a 2017 Spartan 1245 by Prime Time
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| Was a Fulltime Family for 5 years, now we're settlin' down for a spell