Looks like it was a high resistance connection in that junction box. Maybe a loose hold down nut. The TV's pigtail wires are fused in the major fuse box (normally in the engine compartment) and the travel trailer is fused on the travel trailer side under the frame near the battery.
That leaves the junction box vulnerable to a loose wire. If this was the failure mode; once the loose wire starts sparking, the burnt connection gets worse and the resistance goes up. Higher resistance turns that amperage into heat. The entire wire carries that heat and melts the insulation.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW