The topic has been discussed a number of times. Unfortunately, the search tool won't bring it up because "bed" is too short, and "weight" brings up a thousand posts on weight of the TM as a whole.
The real practical answer is "They will support more than you and your friends can ever pack in there."
Here's an example - other threads reach the same conclusion:
http://www.trailmanorowners.com/foru...ead.php?t=6206
Bill