1500 watt heater 1500w divided by 120v come out to 12.5 amps
UL rateing min 16 gage wire under 50 foot
the 50 foot is the total wire length. say 20 foot already on trailer you could use a 30 foot 16 gage to run the heater thats min rated
going over 50 foot rudeces the capacity by approxomently 20%.
14 gage would be a little under rated so next size would be 12gage. for your loads <15 amps i would not go over 50 foot extention that is 12 gauge. Take a look at the cord you are thinking of buying it will have rateings on it. I found someone that explains it better then my cobwebed noodle can