The Kill-A-WATT meter will tell you how much you are using over time.
Are you keeping the extension cord connections dry? Dehumidifiers will use power similar to an AC. Do you have a dead or weak battery? A bad battery will draw constant power and you would never know unless you measure it.
Offer half of the electric bill to keep peace in the family regardless how much you are using. Are you parked there for free using their other utilities and appliances? Water is not free either. Half of the light bill seems inexpensive considering the alternatives.