Rates usually also vary either by the time of day, or by a baseline. On a time-of-day system, the rate will change based on peak usages during the day e.g. it's more in the mornings (6am-9-pm), and more in the evenings (4pm-9pm). On a baseline, you are given a certain amount of kWh usage per month; your bill should specify the amount if you are on that system. If you exceed that, your rates may increase significantly once you exceed 100% of the baseline; the rates then become tiered: e.g. 100%-130%, 131%-200%, 201-300% and each range in excess of the baseline would be an additional charge. I'm on a baseline system, and I was utilizing an electric heater (they're pretty much all 1500W) fairly liberally and I reached the 201%-300% baseline at $0.36 per kWh.
You can check your usage by reading your meter.
Take a reading without the trailer plugged in and do the things you usually do; 24 hours later take another reading and subtract the difference. This amount will be your daily power use in kWh; you can multiply this amount by your rate and then by the number of days in the billing cycle to estimate your monthly bill without the trailer.
Then do the same thing with the trailer plugged in. You can subtract out your average day kWh use to find out how much per day the trailer will cost you. This link will help you read your meter: How to Read Your Electric Meter
Word to the wise... get that furnace up and running and your bill won't hurt much.