There is an online owners manual you can download from this page. I've never seen or heard of a parts list or parts diagram for the coach itself. Your individual appliance manuals often will have them. If you need appliance/subsystem manuals, browse the Manuals section of the forum to download the appliance and subsystem manuals you need.
What, exactly, is the part that you are trying to find? There is no manual for the trailer. There are manuals for the individual items such as the refer, fridge, stove etc. If it is a part of the trailer body you will probably have to call Heartland with your VIN to track down the part. Such as, a fender skirt. These trailers are made up of parts from numerous manufacturers. If you check in the "Tools" section at the top of the page you will find some manuals for individual items.
There is a booklet for the trailer that has general operation information.