Counter weight for added back storage area/deck

I have a 2009 Wildwood X-Lite Backpack edition and it came from the factory with a folding cargo tray. I haven't used the tray yet but plan to this summer. The trailer has a front water tank and the trailer pulls better with about 15 gal of water in the tank. We plan to put our lawn chairs, barbecue, screen house, camp table and folding camp kitchen in a box on the cargo tray. The tray says Max Cap 400 lbs. I just assumed Wildwood would have accounted for the tray capacity in their design.