The tow rating should be similar to a C/K3500 pickup of the same vintage with the same engine, transmission, and rear end ratio.
According to Trailer Life, in 2003 and 2005 (no 2004 info on their website), a G3500 with a GVWR with the 6.0L and 4.10 and a GVWR of 16,000 lbs. should have a tow rating of between 7,000 lbs. and 10,000 lbs.
Of course, the total GCWR of between 23K and 26K has to include the weight of the bus and everything and everybody on the bus.
Your limiting factor is going to be the rear axle GAWR (gross axle weight rating) and the amount of weight your tires can handle.
The 4L80 or 6L80 transmission is plenty stout enough to handle the weight but if your bus doesn't already have a transmission temperature gauge you would be smart to invest in one fairly soon.