Re: Not sure why the negative looks like this
If this was my rig I would (In order of least expensive to most expensive) 1) Go over the metal connecting surfaces with a wire brush to establish bright metal (no poor conducting oxidation ) surfaces.
2) Spray the conducting surfaces with Caig Cramolyn DeOxIt contact restorer/cleaner.
3) Replace the entire negative cable, opting for new one that would connect to the larger negative battery post (not damaged, larger electrical contact area). Cable corrosion and broken individual cable wires in the old cable may be hidden under the shrink tubing.
After doing this repair I would occasionally feel (or use an IR thermometer) on the battery contacts for heating. Most all of the time discharge/charging currents at the battery are slight, so not much heating/contact deterioration would be expected. High 12 volt current use would only normally be expected when parking/leaving and using the landing gear motors and the slideout movement motors. You might want to check if your emergency brake lanyard switch has been activated, and remember that a good battery/wiring system powers the electric brakes to stop your trailer in the event of an on-the-road accidental trailer disconnect.