How big can I go? Taking our time for a new 5th or TT.

We had a 2001 Chevy Silverado 2500 with 6.0 liter, 4.10 rear end,ex cab.
We found that the truck handled the 5er in my signature that weighted in at approx. 9500 lbs loaded. On the flat we were fine power wise, but mountain passes and steep uphill grades were a challenge almost to the extent that it was not very pleasurable towing.
Weight wise there seemed to be no concerns.