Pam,
The hitch on a 1999 Monterey is rated at 5,000 pounds. I would not recommend exceeding that weight without reinforcing the hitch, because I have seen several of them crack to the point that they were dangerous.
The suspension will handle all of the weight that you can add to it. However, the brakes will need supplemental brakes on the vehicle that you are towing to stay within a safe braking distance.
The last concern is the engine and cooling system. The 3126 engine should be able to handle the increased load under almost any condition as long as you are not in a big hurry. However, the cooling system on most 3126 equipped coaches is not oversized. So under a heavy load like a steep grade, you will need to down shift the transmission to keep the engine RPMs around 2000 or a little higher to provide adequate cooling.
Gerald