Welcome to the forums and congrats on getting your new Montana! I'm sure you will enjoy it.
Here's a link to the owner's manuals from Keystone you can down load. Unfortunately, they do not have the 2004 version, but they do have the 2005 version. FORTUNATELY!!! those manuals don't change (hardly any at all) from year to year. If you download the 2005 manual, you'll have everything you need for your 2004 model.
Click here for Keystone's manuals
A bit of advise about your camper. If the camper has been sitting all this time, the most important thing you can do is get new tires on it. Unless the tires have been replaced in the last year or so, they most likely will be dry-rotted, even though the tread may look ok. Dry rot is identified by cracks on the side-walls of the tires. If yours had cracks, even small ones, or even one, replace the tires and don't look back. The last thing you want is a blow-out on your first trip.
Next, if the camper has been sitting all this time, the wheel bearings really should be repacked.
Next, because it's been sitting for so long, you really need to check the brakes also (hitched up and actually moving). Your tow vehicle should be equipped with a brake controller and you need to test it out before hitting the road.
Your RV needs to be road worthy. That is more important than the aesthetics inside the camper. So, make sure it absolutely is functional for road travel. Leave nothing unchecked.... lights, running lights, turn signals, tires, hitch system, everything required for road travel.
Also, make sure your holding tanks are functioning properly also. The last thing you want or need is to discover your black (toilet) holding has a leak, or the slide dump valve is frozen shut from not being used.
One more item that needs to be checked and inspected closely, is the roof. If the calking on the roof has cracks in it, it needs new calking. Dicor calking for RV roofs is probably what you need if your roof is rubber. Also, check the actual rubber roof and make sure there are no tears, pin holes, or wear spots. Rubber roofs of that vintage were designed to last 10 years. Being 20 years old, you really need to inspect it carefully. Water intrusion from the roof will destroy a camper very quickly. Water will weaken the side walls and the floor, and it doesn't take much for damage to occur, especially if the flooring and such was installed with particle board (which was typical for that vintage also). Water is your enemy on any camper!
Here's another internet page I found that you might enjoy also:
Click here.
Again, welcome to the forums and I certainly hope you have a fantastic time with your first journey. And hopefully, all these things I mentioned check out just fine for you. But please, do check these items. Most folks with new campers don't consider these until damage occurs. So, be pro-active here and you'll have great success with your new camper!
Happy camping!