You may find everything you need here:
You can do pretty much anything you want....but be sure you have at least as much square inches of outlet at the furnace as the install manual calls for. In other words put in no more register shutoffs than will make it possible to restrict minimum flow.
Study your furnace and see if there are extra "blank" hookup locations available. You might not want to rob the one duct run for the pipe heater. It may make the outlet for that run not have enough volume and create a cold spot in the trailer.