Is he asking you to drive or park the trailer underneath the wire? Sounds like it's service wire to the house. It's probably 3 (or 4) wires twisted, but older home could mean 3 (or 4) single, bare wires? The twisted is insulated excepted for the bare neutral/support conductor which isn't supposed to be energized. Even if it's bare wires, the bottom wire should be the neutral.
18" clearance from anything on the trailer is sufficient for service wire, hot or not. Most utilities demand that spec. That's measured from the wire to the gutter, eve, metal roof, etc.
Saying all that, what Old-Biscuit says is what the NESC Guidelines suggest. His clearances will keep you or the buyer out of liability issues. Your line to the new owner should be "I'll put it there if you sign this release of my liability." That should get a "Huh, Well...".
Greg
N5LFH
2007 Chariot