Black wire attached to hot water element burnt out

Most likely a low voltage situation occurred which caused a spike in amperage draw. This is a guess, otherwise a lose connection causing a similar situation. Wire is rated for amperage and too high causes resistance which converts to heat.
It's hard to guess on where the wire originates, it'll just be a matter of getting behind the water heater and tracing it.
Good luck, hopefully other Monaco owners might have a better idea of tracing the wire.