Restroom trailer for December 2026 corporate event: heating options?

We're hosting a corporate holiday party outdoors in December 2026. Expecting 150 guests over 5 hours with dinner and an open bar. The site is a paved lot with good truck access for delivery and service. We're renting a 4-station restroom trailer with flushing toilets and sinks to keep things comfortable. My main concern is keeping it warm, December nights here can drop below freezing. I know restroom trailers often have built-in heating, but I'm not sure what methods are reliable for extended use in cold weather. Do most trailers use electric heaters, propane systems, or something else? And what should I confirm with the rental company about winter operation to avoid surprises?