GFCI at shore power tripping

I had an almost identical issue a few months back but without the water heater element. For no reason that I know of, the GFCI I had been using for almost a year, popped as soon as I plugged in. I was using a 50 A to 30 A to 20 A dogbones. I finally plugged it in to another outlet, (non GFCI) and there was no problem. Never figured out why, but I was told RV's and GFCI's typically do not play well together and to avoid using one if possible. I have been on the road since this episode, but expect I will have to replace outlet when I return.