Did you use the same length extension cord with the new trailer that you did with the old one? Even if you have a cord that is heavy enough gauge, you don't want to use one that is any longer than absolutely needed. There is more loss in a long cord vs a short one and if you were marginally ok with a short one, the longer cord could cause enough voltage drop to cause your problem.
2006 GMC 2500HD CCSB 4x4 Duramax/Allison, Titan 52 gallon fuel tank, Prodigy Controller, B&W Companion Hitch
2010 Jayco Designer 35RLTS, Cummins/Onan RV QG 5500 EVAP, Progressive Industries EMS-PT50C, TST Systems 507 TPMS, RV Flex Armor Roof