Do you remember the color of the wire?
If it was white - it is almost always because it was loose so I would be stumped. If it was red or black, then that leg has probably been running overloaded.
I had a 2014 with an Onan 5500 (I really hope it wasn't mine!!, but I don't recognize the name). When I put the progressive dynamics surge suppressor in there, I was able to read how many amps were used on each leg, and it was balanced pretty well, but that doesn't mean they are all the same. You could have all the high draw stuff on one leg, and the other is lightly loaded.
I would definitely install a permanent surge suppressor just to see what is going on and be able to monitor it. If you see higher draw on one leg vs the other, you can balance it better by switching swapping out breakers to balance it out.
Brad & Dory - Lone Tree, CO
CURRENT - 2013 Itasca Meridian 42E/2013 Wrangler 4dr Sahara
SOLD - 2014 Redwood 36RL/2014 F350 DRW