The dealer is right about it being part of the slide, but if they implied that is normal, they are very wrong! As mentioned already, that is the harness for 12V and 120V wiring to your lights and outlet over the drawer. Also, as mentioned, it should be held by a stretch cord, which is likely broken or unattached. The cord should hold the harness somewhat tightly as the slide goes out and back in. On my unit, the cord was so tight it pulled the harness out through the hole in the underside of the slide, pulling the wires out of the shield and fraying the insulation on the wires. I had to re-work the stretch cord and fasten it so the tension was sufficient but not extreme.
If left as you see it, you will have damage to the wires from the slide going out and in.
Tell the dealer it needs to be fixed, and if you buy the unit, check it carefully by looking under it while retracted and also extended.
2020 Eagle HT 30.5 CKTS Modern Farmhouse (16" wheels, spring shackles set on lower hole of frame, SumoSprings TSS-107-40, standard kingpin, B&W Companion slider, TST-507 TPMS)
2019 F250 SRW SuperCab 4X4 SB 6.2 (10,000# GVW) 3.73 Axle 18" wheels