Heat tape will not use to much power. It is probably only about 60 watts total.
1,500 watt heater running 20 hours per day is 30 KW daily, and X 30 days is 900 KW. If they pay a rate say $0.14 per KW for the first 1,000 KW, and more per KW beyond the average power bill, then the price might have been up at 0.18 or 0.21 per KW beyond 1,500 KW used that month. So a typical bill might be say 900 KW, and you add 900 KW to that and get into the higher rate per KW, you can see a significant rate increase.
Of course next year stay in a nearby campground.
The refrigerator is also a 350 watt load, and depending on the temperature it can consume 3KW to 5 KW daily. This would be an additional 90 - 150 KW.
Then if you ran the electric water heater - it has a 800 watt element that might consume 3 KW daily.
The dryer and electricity for the washer can be 3 KW to 5 KW each load, depending on the length of time the heater runs while drying the clothes.
If you are showering in the home, and using a electric water heater, this can consume 4.5 KW per hour it runs, and it can take 3KW to make up for each 10 minute shower at 2 GPM.
Add to that a few extra lights run all the time because visitors are there, the TV is left running more hours each day, ect. and the bill went up.
Good luck,
Fred.