Managing several parks I have seen this multiple times. I have had supposed GFI strips,converters,and bad elements in water heaters cause this. In most cases the RV’er had no idea they were the cause. This can happen in older rigs, not so well maintained rigs as well as new ones. I always explain to the RV’ers that use the GFI strips if they are so great why dosent the industry build them into the rigs. I do not excuse bad wiring in some parks and so called know everything maintenance guys. But I personally have only found this to be the case in 1 park.