I have an '83 k10. With a 3.08 rear gear, it seemed to start to get unhappy if the gross load (truck and trailer) got over 10,000#. Around my property I sometimes "dolly around" 12,000# 5th wheel trailers with no issues.
At the end of the day, maximum load is kind of limited by several things, including but not limited to:
1) Rear gear ratio, bigger gear numbers help bunches to move bigger loads.
2) Terrain - any hills along the way?
3) Trailer brakes - I am a big fan of some extra stoppers out back when towing; have had too may close calls without them.
4) How big a hurry are you in? Are you moving fast down the expressway or just putting along on a farm-market blacktop? Moving fast means more hp required, it all going thru the gear boxes out the rear end.
5) How far are you going? across a small town or across several states?