Well, here are a couple examples. We have been on metered electric 3 x over the last several years.... one was 14 cents a kw. Our electric bill then, in a 5th wheel, all appliances, and a/c, was $54 for a month.
Another time at 10 cents a kw, in our DP, using electric space heaters, elec water heater, heat pumps some time, and all other appliances was $38.
Finally, one week, in our DP, using a/c, and all appliances was $8.
The only way you will know, is, as others suggested, use a KW meter.
I feel for you. A couple years ago at Christmas, we were plugged into a relatives for a week. It just so happened to be the coldest winter in the area, and there was snow on the ground. My MIL keeps the house at 75, but she blamed us for the huge electric bill that month. We knew it was a crock, but to keep the peace, we gave them $65, and put it behind us.
The heated baseboards in their house, plus it being their first Winter, she was probably just shell shocked at the bill, and looking for blame.
If you only used the appliances you stated, your portion, for the 5th wheel, should be around $20, as a guesstimate.
You do have to factor in all the other amenities.... If you are using their shower, washer/dryer, kitchen, etc, all that adds up.
Good luck to you. I hope you can work it out. It sounds like pretty cheap storage, and if you are using their house too, they are doing you a huge favor.
I would suck it up, and pay her whatever she wants.