Ultimately, the maximum weight will depend on:
1. The GVW you paid for on the truck's registration.
2. The GVW you paid for on the trailer's registration.
3. How slow you want to go up hills.
With a truck that has a GVWR of 13,500lbs, and a trailer with a GVWR of 20,000lbs, if you register them both for the maximum, and you have a CDL, you can LEGALLY go down the road at 33,500lbs.
You won't be exceeding any of the truck's ratings, except for the GCVWR. The GCVWR is generally, unofficially, regarded as a "performance" rating because it can be exceeded without exceeding any other rating.