The Crossfire line is a light weight camper with a light weight frame under it. At almost 20' and weighing only 3400 pounds, there may not be sufficient strength in the frame rails to support any additional weight (receiver and cargo tray) behind the extreme rear of the frame rails. I'd urge you to consult an experienced welding shop and discuss your plans and your needs. Be very explicit with them about what you want to put on the cargo tray so they know what you're expecting.
Keep in mind that any alterations to the frame will void your Keystone warranty, so proceed with that fact in mind as you make plans. Keystone won't refuse warranty repairs to items that are not affected by the modification, so you'll be OK if you need repairs to things inside the trailer, appliances, etc, but you may find that Keystone will refuse to repair anything that's associated with the frame, things like hitch, walls, windows, bed slide mechanisms, doors and roof structure come to mind.
I would urge you to proceed with caution, your trailer probably isn't engineered to support very much weight on the rear. With a 1300 pound cargo maximum, hanging 200 or 300 pounds off the rear bumper (or the rear frame hangers) will substantially change your tongue weight, put stress on the frame rails and may cause problems with the single axle design of your trailer.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS