125' would be fine if you define power source as a panel. Grab a 100' of 10/2 romex . Run it out and put a plug (cord cap) on it. Done. The #4 or 6 is  not needed at all unless you are running pipe but in free air (a stretch I know) you are fine. 
Hopefully you know someone who needs the #10 for a dryer or hot water heater later, stuff isn't cheap any more.  I my world I would grab a reel of #10 an just run the wire  out to the trailer on the ground if in a safe place, but then electrician's are allowed to do these things, LOL