You can insure anything....
No need to insure a trailer as most likely your auto insurance will cover the towed vehicle.
A general umbrella policy might be as cheap as $300 per year and give you a million in coverage, which as it sounds, covers everything in your life like an umbrella. The exception is sexual crimes and possibly crimes you purposely committed. This would cover damages your trailer made as well as anything you might get sued for in your daily life. Many variables in different policies, but this is the way to go to protect yourself if you have any assets a lawsuit could come after.