Outdoor GFCI outlet trips when using electric cooktop

My wife's brother and family got rid of the propane outdoor appliances and went to all electric, they set up a table next the trailer and cook away, until a few years ago when it kept tripping the GFCI (it was on bathroom circuit, took them awhile to find it the first time).. they asked me how to figure out what is happening, I said to just run extension cord from power post to cook table.. most power posts have a 20amp outlet next the 30/50 so use it. No more problems..