Get a dump trailer. You'll never regret that purchase. It will make your life so much easier.
Regarding insurance. If you have a private (individual) policy, the tow vehicles insurance will cover the trailer...if it's not being used commercially. If you have commercial insurance on your truck (which is most often cheaper than an Individual policy), that insurance will only cover the truck. With commercial insurance, everything needs a policy. In the event that the trailer became separated from the tow vehicle and caused damage you would be liable - if you didn't have a policy on the trailer.
Design - Build - Restore