I have been evaluating this since your response (given that I will be years away from getting an exepdition trailer to mount the tent on) and I ended up reaching out to Frontrunner to check how much beyond the stated weight-rating one could actually go and this is what they responded with
QUOTE
I can not give you an overload rating, however the Front Runner Slimline II Rack Tray is designed to carry heavy loads and is one of the strongest, if not THE strongest out there. Our engineers have dialed in a safe load rating at 660lbs (300kg) but it has been known to carry more. Have you seen us load 1540 lbs of water on a Front Runner rack mounted to a Land Rover Discovery 4? Check out our YouTube channel.
This rack is so darn strong it can exceed the factory load rating of some vehicle’s roofs. Please contact your vehicle manufacturer to check your vehicle’s specific load ratings. And always load your Front Runner Rack with evenly dispersed weight and lots of common sense!
A good rule of thumb in the industry is your dynamic weight should be about 1/3 of your static weight limit (660lb).
Let us know if you have any other questions,
Alyssa
UNQUOTE
Unless I am mistaken in how these ratings are assessed, I'm guessing you could easily load well over a 1000 lbs on the rack given that the roof is probably designed to hold the entire weight of the truck in the event of a rollover.