It's the reservoir for your Hydro-boost power brake system. It should say Dextron III on the side which is transmission fluid. Your brakes are hydraulic over hydraulic. A pump sends this fluid to your Master cylinder up front to boost your brake pedal pressure, then the fluid returns back to that reservoir. You can sometimes hear a whistle or whine when you are stopped and holding the brake pedal down. This was one complaint some people had with the system. Nothing is wrong with it when it does it, it's just the nature of the beast.
You mentioned heaters and hot water so I suspect you have a Hurricane hydronic heat system? If so, the overflow tank and reservoir for that should be in the engine compartment under the bed mounted to the wall of the bed platform. The reservoir is a small black tank with a radiator cap on it and nearby is a small plastic overflow tank just like a car has where you add coolant.
Fluid in the tank you are asking about should be reddish or reddish brown and smell oily.
Is you'll notice by my signature, I have a '98 Serengeti which should have a lot of similarities to your rig so I'm happy to answer any questions you have. don't hesitate to ask either via the forum, or a private message. There are other Safari owners who post in the Monaco section, and there's a Safari Owner's specific forum off site at www.safarifriends.org