Pony

Took trailer out a few days earlier. Used heater, worked fine. closed all propane tanks and turned off all appliances. Apon returning home from 2 hr drive batteries to unit showed empty from a previous full charge. Plunged trailer into electric to charge and within minutes rotten egg gass smell very string coming from fur nice vent. Unplug electric from trailer outlet smell goes away. Why does heater smell like gass leak but propane tanks are shut off? Only smell when I plug in to charge onboard batteries. No smell inside unit. Let set 24 hrs. Tried to charge same thing Gass smell. Within minutes. What's wrong? Is it safe to towe to repair shop? Yes I have trailer unplugged from electric.