^^^ That's it.
The slideout boxes are attached by bolts to plates only on the outside ends of the slide arms. There are usually rollers on the frame that support the box as it slides out and then support the inner edge of the box when it is fully out. But when the slide is in, the rollers are nearly at the same place as the end plates and any added weight can torque the box quite a bit resulting in damage to the floor where the rollers are or where the box is bolted to the end plates of the slide arms. The reason there aren't rollers on the underside of the inner edge of the box is that they would have to roll across carpet or vinyl causing damage to those surfaces.
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam