Your extension cord is probably not a 12 gauge one and you are drawing enough current through your ext cord to drop the voltage to unacceptable levels as far as the EMS is concerned. Initially the converter will draw several amps until it reaches float charge levels. What voltage does the EMS remote say is coming into the coach with the converter breaker turned ON?
Two ways to solve this: get a heavier 12 gauge cord or bypass the EMS with the switch provided on the EMS remote. As long as the converter is the only thing drawing power, the lower voltage will not hurt it if you have at least 100V.