Lots of possibilities. Process of elimination could take some time.
Are any high amp usage (electric heat, electric water heater, microwave, air conditioner, etc) items being used?
Are other items (power tools, air compressor, other trailers, etc) running on the same circuit?
Are you using an extension cord that is maybe too long, not heavy enough to carry the load, or, not in good condition?
Any electrical connections that might have moisture?
Something plugged into an outlet, in your trailer, that might be shorted or not grounded?
Still could be the outlet.