I agree with the above posts. You may have the problem on the extension cord ends or the connectors the extension cord plugs into.
To check for problem areas, run the hearer at the end for a bit and then turn off the power. Then disconnect the extension cord and with an infrared thermometer gun (laser temp gun) read the temperature at each of the four connection points:
-RV end of extension cord,
-the source end of extension cord and
-the source/supply receptacle.
Check the sides and at the connection pins. Any temperature that is noticeably higher that the others is the suspect connector and it may need replacing.
Most likely source of the problem is the connector on the RV or at the electricity supply point. I'd be especially suspicious of a supply connection that is outside in the weather even if it is a "waterproof" box.
2000 Alpine Coach 36' FDS, Cummins ISC 330