I believe the issue was that it was a 240 welding outlet wired without a neutral or the neutral was corroded or bad. Since a 240 welder does not use a neutral, the circuit would not show as bad when the welder was used.
240 is NOT 240 when you actually need 2 phases of 110.
The camper is not wired for 240; it uses two 120 volt circuits. Those two 120 volt circuits require a solid neutral to complete the 2 120 volt circuits. Without a neutral, the converter panel will make a 240 circuit through the neutral buss inside the converter (where all the camper's white wires are connected). This will put 240 volts through any 120 volt items connected and (sometimes) on when power is applied.
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