Trailer rentals nationwide??

My first thought was Star Leasing but they are only in the eastern US..

 

extra trailer rents and leases but mostly long term

 

Most rental companies will not do that, however Xtra Lease does, and they will also let you "load out" trailers, where you get cheap or free use of their trailer to help them relocate it to other branches in need of a specific trailer type. It really is a pretty sweet deal most of the time, as long as you can find freight into and out of the areas you need to be while servicing their branch locations.
http://www.xtralease.com/Rent-Lease-Or-Buy/Traffic/Pages/OneWayRentals.aspx

You could also look into new trailer delivery, most of the time the manufacturers will also let you "load out" a trailer with very generous terms. Typical delivery of a new dry van will let you have use of the trailer for up to a week where you can load it with your freight enroute to the dealer of customer. Just be careful not to damage it or you now own a brand new trailer. I don't know if flat deck manufacturers will do this, but it is something to look into since you are thinking outside the box.