Check that site out. GVWR doesn't refer to the bus weight, it refers to the total max weight. You weigh in at 8800, so you have 1200 available if you don't want to exceed the vehicles engine, tranny and brake design specs.
So your trailer and load can't exceed 1200 lbs.
You need to weigh your bus by the axles to learn your GAWR. Subtract your actual rear wheel weight from 7500, that gives you the max tongue weight.
As long as your trailer and load are less than 1200 lbs, use 10-15% of trailer and load weight to calculate tongue weight. Let's say your trailer and load weigh in at 1000 lbs; your test is GAWR minus the Actual Axle Weight minus ~150 lbs. As long as the resulting number is positive, you meet the design guidelines and can tow.