Could it be wet carpet, wet plywood in slide corner areas as some have previously mentioned on here? Perhaps AC gaskets on roof. You could pull covers easily and check for duct integrity at same time...
Mike & Pam '12 36RL, Kodiak Discs, 17.5" G114s H-Rated, SRE4000 w/HD Shackles, Joy Rider Shocks, Flex Air, 5.5 Onan, W&D, DW, 4 Scare Lights, (2) 6V Trojan T-145s; Progressive EMS-HW50C, Winegard DTV Sat, Sony/Denon 4K Home Theater, WiFiRanger EliteAC Pack, weBoost 4G-X Cell Booster.
2020 RAM 3500 Limited CC, LB, 4x4, DRW, Max Tow, Cummins HO, AISIN, 4.10, Mopar 30K w/OEM Prep, Transfer Flow 70gal Aux Tank/Toolbox.