I never loan out any expensive or hard to operate tools to family or friends. I do the work for them and ask for a root beer as payment. Be honest and up front with them.... something like:
I am nervous about loaning out my truck and trailer due to liability and damage. If you hurt someone else with my equipment, I may get sued. If you damage it, I will feel bad and it may put a strain on our relationship. However, if I hurt someone else or damage it, then it is my problem. Let's pick a better time when I can help you. If we can't find a good time, perhaps you could rent something to do the job and I will pitch in to help you with the costs.
That would seem gracious and reasonable, in my opinion. Best of luck.
