Tow rating, GMC Topkick

I have a 1990 GMC Top Kick 7000. It has Cat 3208 10.4 litre turbo V8, Allison transmission. GVWR is 28000. It has air brakes. Current trailer is electric brakes, but I'd need a different trailer anyway.
What are the limits I could legally tow, or safely tow, whichever is less. It'd be 3 miles of reasonably level 50 MPH speed limit highway between two parcels I own. I don't mind driving 20 MPH for this. I'm tempted to buy an old excavator that weighs 16 tons.
If I have to depend on a mover,...

Tow rating, GMC Topkick

Tow rating, GMC Topkick