If you do the work yourself I doubt you could "insure" that time for
reimbursement. By the same token I doubt money paid for labor to others
will add to the "value" of any insured item UNLESS you have a rider policy
that calls for "replacement" without further cost to you.
Either way, if your trailer gets destroyed and the insurance company
"replaces" what you've lost it will still be an old, vintage trailer that
they find for you; or one that you find for you and let them purchase for
you.
But there is no value put on labor you do yourself, the labor of others to
rebuild, maybe.
Tom