My mother in-law was saying how she heard that when you plug into a normal house outlet you use 3x as much power or something.
It is hard to argue with a statement like that, which contains no factual information at all. Your MIL may have a totally different grievance that she can not / will not address, and this is the "outlet" she chose.
Or the "spike" they observe is always there in the winter - they just never looked in the past when your camper was not there. We use natural gas for heating and hot water. The consumption of January can be 30x the reading of July.
The previous poster's suggestion to add your own meter may not help whan your in-laws want to believe otherwise. But it will give you an exact knowledge of how much you actually use.
Do your in-laws get electric bills that state the actual consumption and the meter readings at the time? You should be able to compare the current usage to last year's, when your camper was not there. At least you should be able to calculate a daily average, which you can easily compare with current meter reading. Then unplug for a day or two...