How much electiticy costs to run small RV heaters?

I hope you had a contract when you moved onto the property. In the contract should have been how the electric costs were are covered. If the electric costs are included in the rent for the lot, and the price is fixed, and there no clause in the contract for going over an average usage, then I'd tell him to go take a hike.

However, if there is no contract or nothing is stated, then I think you should offer to help pay for your share. Get it in writing and keep receipts.