I had a similar problem with my 5th wheel.
Under my pin box (outside) there is a red covered current limiter/isolator (40 amp) that isolates the 12 vdc of the camper from the 12 vdc of the truck, yet allows charging of the camper battery when the truck's voltage exceeds camper battery voltage when connected.
My problem was solved by removing the wire round terminal ends from the isolator threaded terminals, sanding the corrosion off them, and reinstalling.
That corrosion prevented current flow from the converter to the battery buss which contains the battery, the landing gear fuse, and my main slide fuses (in my camper).
Lou, Laura, & Freya the wonder dog
2008 GMC Sierra 3000HD Crewcab SB Allison Duramax
2019 Flagstaff 8529FLS - Pullrite 3300
HAM CALLSIGN - KC3FFW