Steve answered the form question but how much slope are we dealing with from one end of the bathroom to the other? Inches or is it such a gradual slope that you won't notice it anyway with carpet? If it isn't that noticable, then shooting for perfection may be more trouble than it is worth.
15' is a long way to try to level. If there is so much that you are talking forms, then you need a couple guys mixing 5 gallons buckets of the stuff up while you are pouring and working it. It sets up too fast to do a big job by yourself.
If you are talking an inch or more from one end to the other(since you are mentioning forms), and it is noticable, then use a very dry mortar mix like is done for shower bases and building up floors for tile. It is basically mixed very dry, works with a float and screed, and is slow in setting up.